<PAGE>
                                                                     EXHIBIT 3.1


                     RESTATED CERTIFICATE OF INCORPORATION
                                       OF
                               UNOCAL CORPORATION
                  (Originally incorporated on March 18, 1983)


       FIRST:  The name of this corporation is:

                               UNOCAL CORPORATION

       SECOND:  The name and address of the registered agent of the corporation
in the State of Delaware is:

                         The Corporation Trust Company
                            Corporation Trust Center
                               1209 Orange Street
                 Wilmington, New Castle County, Delaware 19801

       THIRD:  The purpose of the corporation is to engage in any lawful act or
activity for which corporations may be organized under the General Corporation
Law of Delaware.

       FOURTH:  The total number of shares of stock which the corporation shall
have authority to issue is eight hundred fifty million (850,000,000) shares,
consisting of seven hundred fifty million (750,000,000) shares of Common Stock,
having a par value of $1.00 per share, and one hundred million (100,000,000)
shares of Preferred Stock, having a par value of $0.10 per share.

       The board of directors is authorized, subject to any limitations
prescribed by law, to provide for the issuance of the shares of Preferred Stock
in one of more series, and by filing a certificate pursuant to the applicable
law of the State of Delaware, to establish from time to time the number of
shares to be included in each such series, and to fix the designation, powers,
preferences, and rights of the shares of each such series and any
qualifications, limitations or restrictions thereof.  The number of authorized
shares of Preferred Stock may be increased or decreased (but not below the
number of shares thereof then outstanding) by the affirmative vote of the
holders of a majority of the Common Stock, without a vote of the holders of the
Preferred Stock, or of any series thereof, unless a vote of any such holders is
required pursuant to the certificate or certificates establishing the series of
Preferred Stock.

       Pursuant to the authority vested in the board of directors by the
preceeding paragraph of this Article FOURTH, the following series of Preferred
Stock has been created, and the designation and amount thereof and the voting
powers, preferences and relative, participating, optional and other special
rights of the shares of such series, and the qualifications, limitations or
restrictions thereof are as set forth in the exhibit attached hereto as
specified below and incorporated herein by reference:

       Exhibit I    Series B Junior Participating Preferred Stock
<PAGE>

       FIFTH:  New bylaws may be adopted or the bylaws may be amended or
repealed by a vote of seventy-five percent of the outstanding stock of the
corporation entitled to vote thereon.  Bylaws may also be adopted, amended or
repealed by the Board of Directors as provided or permitted by law; however, any
bylaw amendment adopted by the Board of Directors increasing or reducing the
authorized number of directors shall require a resolution adopted by the
affirmative vote of not less than seventy-five percent of the directors.

       SIXTH:  The number of directors which shall constitute the whole Board of
Directors of the corporation shall be as specified in the bylaws of the
corporation, subject to the provisions of Article FIFTH hereof and this Article
SIXTH.  The board is divided into three classes, Class I, Class II and Class
III.  Such classes shall be as nearly equal in number of directors as possible.
Each director shall serve for a term ending on the third annual meeting
following the annual meeting at which such director was elected; provided,
however, that the directors first elected to Class I shall serve for a term
ending on the annual meeting next following the end of the calendar year 1983,
the directors first elected to Class II shall serve for a term ending on the
second annual meeting next following the end of the calendar year 1983, and the
directors first elected to Class III shall serve for a term ending on the third
annual meeting next following the end of the calendar year 1983.  The foregoing
notwithstanding, each director shall serve until his successor shall have been
duly elected and qualified, unless he shall resign, become disqualified,
disabled or shall otherwise be removed.

       At each annual election, the directors chosen to succeed those whose
terms then expire shall be of the same class as the directors they succeed,
unless, by reason of any intervening changes in the authorized number of
directors, the Board shall designate one or more directorships whose term then
expires as directorships of another class in order more nearly to achieve
equality of number of directors among the classes.

       Notwithstanding the rule that the three classes shall be as nearly equal
in number of directors as possible, in the event of any change in the authorized
number of directors each director then continuing to serve as such shall
nevertheless continue as a director of the class of which he is a member until
the expiration of his current term, or his prior death, resignation or removal.
If any newly created directorship may, consistent with the rule that the three
classes shall be as nearly equal in number or directors as possible, be
allocated to one or two or more classes, the Board shall allocate it to that of
the available classes whose term of office is due to expire at the earliest date
following such allocation.

       SEVENTH: The affirmative vote of the holders of not less than seventy-
five percent of the outstanding stock of the corporation entitled to vote shall
be required for approval if (1) this corporation merges or consolidates with any
other corporation if such other corporation and its affiliates singly or in the
aggregate are directly or indirectly the beneficial owners of more than ten
percent (10%) of the total voting power of all outstanding shares of the voting
stock of this corporation (such other corporation being herein referred to as a
"Related Corporation"), or if (2) this corporation sells or exchanges all or a
substantial part of its assets to or with such Related Corporation, or if (3)
this corporation issues or delivers any stock or other securities of its issue
in exchange or payment for any properties or assets of such Related Corporation
or securities issued by such Related Corporation, or in a merger of any
affiliate of this corporation with or into such

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<PAGE>

Related Corporation or any of its affiliates; provided, however, that the
foregoing shall not apply to any such merger, consolidation, sale or exchange,
or issuance or delivery of stock or other securities which was (i) approved by
resolution of the Board of Directors adopted by the affirmative vote of not less
than seventy-five percent of the directors prior to the acquisition of the
beneficial ownership of more than ten percent (10%) of the total voting power of
all outstanding shares of the voting stock of the corporation by such Related
Corporation and its affiliates, nor shall it apply to any such transaction
solely between this corporation and another corporation fifty percent (50%) or
more of the voting stock of which is owned by this corporation. For the purposes
hereof, an "affiliate" is any person (including a corporation, partnership,
trust, estate or individual) who directly, or indirectly through one or more
intermediaries, controls, or is controlled by, or is under common control with,
the person specified. "Control" means the possession, directly or indirectly, of
the power to direct or cause the direction of the management and policies of a
person, whether through the ownership of voting securities, by contract, or
otherwise; and in computing the percentage of outstanding voting stock
beneficially owned by any person the shares outstanding and the shares owned
shall be determined as of the record date fixed to determine the stockholders
entitled to vote or express consent with respect to such proposal. The
stockholder vote, if any, required for mergers, consolidations, sales or
exchanges of assets or issuances of stock or other securities not expressly
provided for in this Article, shall be such as may be required by applicable
law. A "substantial part" of the corporation's assets shall mean assets
comprising more than ten percent of the book value of fair market value of the
total assets of the corporation and its subsidiaries taken as a whole.

       EIGHTH: No action shall be taken by the stockholders except at an annual
or special meeting of stockholders. No action shall be taken by stockholders by
written consent.

       NINTH: Special meetings of the stockholders of the corporation for any
purpose or purposes may be called at any time by the Board of Directors, or by a
majority of the members of the Board of Directors, or by a committee of the
Board of Directors which has been duly designated by the Board of Directors and
whose powers and authority, as provided in a resolution of the Board of
Directors or in the by-laws of the corporation, include the power to call such
meetings, but such special meetings may not be called by any other person or
persons; provided, however, that, if and to the extent that any special meeting
of stockholders may be called by any other person or persons specified in any
provisions of this Certificate of Incorporation or any amendment thereto, then
such special meeting may also be called by the person or persons, in the manner,
at the times and for the purposes so specified.

       TENTH: The corporation reserves the right to amend, alter, change or
repeal any provision contained in this Certificate of Incorporation, in the
manner now or hereafter prescribed by statute, and all rights conferred on
stockholders herein are granted subject to this reservation. Notwithstanding the
foregoing, the provisions set forth in Articles FIFTH, SIXTH, SEVENTH, EIGHTH,
NINTH and this Article TENTH may not be repealed or amended in any respect
unless such repeal or amendment is approved by the affirmative vote of the
holders of not less than seventy-five percent of the total voting power of all
outstanding shares of voting stock of this corporation.

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<PAGE>

       ELEVENTH:  A director of the corporation shall not be personally liable
to the corporation or its shareholders for monetary damages for breach of
fiduciary duty as a director, except for liability (i) for any breach of the
director's duty of loyalty to the corporation or its shareholders, (ii) for acts
or omissions not in good faith or which involve intentional misconduct or a
knowing violation of law, (iii) under Section 174 of the Delaware General
Corporation Law, or (iv) for any transaction from which the director derived an
improper personal benefit.  If the Delaware General Corporation Law is amended
after approval by the shareholders of this article to authorize corporate action
further eliminating or limiting the personal liability of directors, then the
liability of a director of the corporation shall be eliminated or limited to the
fullest extent permitted by the Delaware General Corporation Law, as so amended.

       Any repeal or modification of the foregoing paragraph by the shareholders
of the corporation shall not adversely affect any right or protection of a
director of the corporation existing at the time of such repeal or modification.

       IN WITNESS WHEREOF, this Restated Certificate of Incorporation, which
only restates and integrates and does not further amend the provisions of the
corporation's Restated Certificate of Incorporation as heretofore amended or
supplemented, there being no discrepancy between those provisions and the
provisions of this Restated Certificate of Incorporation except as permitted by
Section 245 of the General Corporation Law of Delaware, having been duly adopted
by the corporation's Board of Directors in accordance with Section 245 of the
General Corporation Law, has been executed by its duly authorized officer as of
the 31st day of January, 2000.


                              UNOCAL CORPORATION



                              By:  /s/ DENNIS CODON
                                   ----------------
                                  Name:  Dennis P.R. Codon
                                  Title:    Vice President

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<PAGE>

                                   EXHIBIT I

                 SERIES B JUNIOR PARTICIPATING PREFERRED STOCK


          Section 1.  Designation and Amount.  The shares of such series shall
                      ----------------------
be designated as "Series B Junior Participating Preferred Stock" (the "Series B
Preferred Stock") and the number of shares constituting the Series B Preferred
Stock shall be 5,000,000.  Such number of shares  may be increased or decreased
by resolution of the Board of Directors; provided, that no decrease shall reduce
                                         --------
the number of shares of Series B Preferred Stock to a number less than the
number of shares then outstanding plus the number of shares reserved for
issuance upon the exercise of outstanding options, rights and warrants and upon
the conversion of any outstanding securities issued by the Corporation
convertible into Series B Preferred Stock.

          Section 2.  Dividends and Distributions.
                      ---------------------------

          (A) Subject to the rights of the holders of any shares of any series
     of Preferred Stock (or any similar stock) ranking prior and superior to the
     Series B Preferred Stock with respect to dividends, the holders of shares
     of Series B Preferred Stock, in preference to the holders of Common Stock,
     par value $1.00 per share (the "Common Stock"), of the Corporation, and of
     any other junior stock, shall be entitled to receive, when, as and if
     declared by the Board of Directors out of funds legally available for the
     purpose, quarterly dividends payable in cash on the first day of March,
     June, September and December in each year (each such date being referred to
     herein as a "Quarterly Dividend Payment Date"), commencing on the first
     Quarterly Dividend Payment Date after the first issuance of a share or
     fraction of a share of Series B Preferred Stock, in an amount per share
     (rounded to the nearest cent) equal to the greater of (a) $1 or (b) subject
     to the provision for adjustment hereinafter set forth, 100 times the
     aggregate per share amount of all cash dividends, and 100 times the
     aggregate per share amount (payable in kind) of all non-cash dividends or
     other distributions, other than a dividend payable in shares of Common
     Stock or a subdivision of the outstanding shares of Common Stock (by
     reclassification or otherwise), declared on the Common Stock since the
     immediately preceding Quarterly Dividend Payment Date or, with respect to
     the first Quarterly Dividend Payment Date, since the first issuance of any
     share or fraction of a share of Series B Preferred Stock.  In the event the
     Corporation shall at any time declare or pay any dividend on the Common
     Stock payable in shares of Common Stock, or effect a subdivision or
     combination or consolidation of the outstanding shares of Common Stock (by
     reclassification or otherwise than by payment of a dividend in shares of
     Common Stock) into a greater or lesser number of shares of Common Stock,
     then in each  such case the amount to which holders of shares of Series B
     Preferred Stock were entitled immediately prior to such event under clause
     (b) of the preceding sentence shall be adjusted by multiplying such amount
     by a fraction, the numerator of which is the number of shares of Common
     Stock outstanding immediately after such event and the denominator of which
     is the number of shares of Common Stock that were outstanding immediately
     prior to such event.

          (B) The Corporation shall declare a dividend or distribution on the
     Series B Preferred Stock as provided in paragraph (A) of this Section
     immediately after it declares
<PAGE>

     a dividend or distribution on the Common Stock (other than a dividend
     payable in shares of Common Stock); provided that, in the event no dividend
     or distribution shall have been declared on the Common Stock during the
     period between any Quarterly Dividend Payment Date and the next subsequent
     Quarterly Dividend Payment Date, a dividend of $1 per share on the Series B
     Preferred Stock shall nevertheless be payable on such subsequent Quarterly
     Dividend Payment Date.

          (C) Dividends shall begin to accrue and be cumulative on outstanding
     shares of Series B Preferred Stock from the Quarterly Dividend Payment Date
     next preceding the date of issue of such shares, unless the date of issue
     of such shares is prior to the record date for the first Quarterly Dividend
     Payment Date, in which case dividends on such shares shall begin to accrue
     from the date of issue of such shares, or unless the date of issue is a
     Quarterly Dividend Payment Date or is a date after the record date for the
     determination of holders of shares of Series B Preferred Stock entitled to
     receive a quarterly dividend and before such Quarterly Dividend Payment
     Date, in either of which events such dividends shall begin to accrue and be
     cumulative from such Quarterly Dividend Payment Date.  Accrued but unpaid
     dividends shall not bear interest.  Dividends paid on the shares of Series
     B Preferred Stock in an amount less than the total amount of such dividends
     at the time accrued and payable on such shares shall be allocated pro rata
     on a share-by-share basis among all such shares at the time outstanding.
     The Board of Directors may fix a record date for the determination of
     holders of shares of Series B Preferred Stock entitled to receive payment
     of a dividend or distribution declared thereon, which record date shall be
     not more than 60 days prior to the date fixed for the payment thereof.

          Section 3.  Voting Rights.  The holders of shares of Series B
                      -------------
     Preferred Stock shall have the following voting rights:

          (A) Subject to the provision for adjustment hereinafter set forth,
     each share of Series B Preferred Stock shall entitle the holder thereof to
     100 votes on all matters submitted to a vote of the stockholders of the
     Corporation.  In the event the Corporation shall at any time declare or pay
     any dividend on the Common Stock payable in shares of Common Stock, or
     effect a subdivision or combination or consolidation of the outstanding
     shares of Common Stock (by reclassification or otherwise than by payment of
     a dividend in shares of Common Stock) into a greater or lesser number of
     shares of Common Stock, then in each such case the number of votes per
     share to which holders of shares of Series B Preferred Stock were entitled
     immediately prior to such event shall be adjusted by multiplying such
     number by a fraction, the numerator of which is the number of shares of
     Common Stock outstanding immediately after such event and the denominator
     of which is the number of shares of Common Stock that were outstanding
     immediately prior to such event.

          (B) Except as otherwise provided herein, in any other Certificate of
     Designations creating a series of Preferred Stock or any similar stock, or
     by law, the holders of shares of Series B Preferred Stock and the holders
     of shares of Common Stock

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<PAGE>

     and any other capital stock of the Corporation having general voting rights
     shall vote together as one class on all matters submitted to a vote of
     stockholders of the Corporation.

          (C) Except as set forth herein, or as otherwise provided by law,
     holders of Series B Preferred Stock shall have no special voting rights and
     their consent shall not be required (except to the extent they are entitled
     to vote with holders of Common Stock as set forth herein) for taking any
     corporate action.

          Section 4.  Certain Restrictions.
                      --------------------

          (A) Whenever quarterly dividends or other dividends or distributions
     payable on the Series B Preferred Stock as provided in Section 2 are in
     arrears, thereafter and until all accrued and unpaid dividends and
     distributions, whether or not declared, on shares of Series B Preferred
     Stock outstanding shall have been paid in full, the Corporation shall not:

               (i) declare or pay dividends, or make any other distributions, on
          any shares of stock ranking junior (either as to dividends or upon
          liquidation, dissolution or winding up) to the Series B Preferred
          Stock;

               (ii) declare or pay dividends, or make any other distributions,
          on any shares of stock ranking on a parity (either as to dividends or
          upon liquidation, dissolution or winding up) with the Series B
          Preferred Stock, except dividends paid ratably on the Series B
          Preferred Stock and all such parity stock on which dividends are
          payable or in arrears in proportion to the total amounts to which the
          holders of all such shares are then entitled;

               (iii)  redeem or purchase or otherwise acquire for consideration
          shares of any stock ranking junior (either as to dividends or upon
          liquidation, dissolution or winding up) to the Series B Preferred
          Stock, provided that the Corporation may at any time redeem, purchase
          or otherwise acquire shares of any such junior stock in exchange for
          shares of any stock of the Corporation ranking junior (either as to
          dividends or upon dissolution, liquidation or winding up) to the
          Series B Preferred Stock; or

               (iv) redeem or purchase or otherwise acquire for consideration
          any shares of Series B Preferred Stock, or any shares of stock ranking
          on a parity with the Series B Preferred Stock, except in accordance
          with a purchase offer made in writing or by publication (as determined
          by the Board of Directors) to all holders of such shares upon such
          terms as the Board of Directors, after consideration of the respective
          annual dividend rates and other relative rights and preferences of the
          respective series and classes, shall determine in good faith will
          result in fair and equitable treatment among the respective series or
          classes.

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<PAGE>

          (B) The Corporation shall not permit any subsidiary of the Corporation
     to purchase or otherwise acquire for consideration any shares of stock of
     the Corporation unless the Corporation could, under paragraph (A) of this
     Section 4, purchase or otherwise acquire such shares at such time and in
     such manner.

          Section 5.  Reacquired Shares.  Any shares of Series B Preferred Stock
                      -----------------
purchased or otherwise acquired by the Corporation in any manner whatsoever
shall be retired and cancelled promptly after the acquisition thereof.  All such
shares shall upon their cancellation become authorized but unissued shares of
Preferred Stock and may be reissued as part of a new series of Preferred Stock
subject to the conditions and restrictions on issuance set forth herein, in the
Restated Certificate of Incorporation, or in any other Certificate of
Designations creating a series of Preferred Stock or any similar stock or as
otherwise required by law.

          Section 6.  Liquidation, Dissolution or Winding Up.  Upon any
                      --------------------------------------
liquidation, dissolution or winding up of the Corporation, no distribution shall
be made (1) to the holders of shares of stock ranking junior (either as to
dividends or upon liquidation, dissolution or winding up) to the Series B
Preferred Stock unless, prior thereto, the holders of shares of Series B
Preferred Stock shall have received $100 per share, plus an amount equal to
accrued and unpaid dividends and distributions thereon, whether or not declared,
to the date of such payment, provided that the holders of shares of Series B
Preferred Stock shall be entitled to receive an aggregate amount per share,
subject to the provision for adjustment hereinafter set forth, equal to 100
times the aggregate amount to be distributed per share to holders of shares of
Common Stock, or (2) to the holders of shares of stock ranking on a parity
(either as to dividends or upon liquidation, dissolution or winding up) with the
Series B Preferred Stock, except distributions made ratably on the Series B
Preferred Stock and all such parity stock in proportion to the total amounts to
which the holders of all such shares are entitled upon such liquidation,
dissolution or winding up.  In the event the Corporation shall at any time
declare or pay any dividend on the Common Stock payable in shares of Common
Stock, or effect a subdivision or combination or consolidation of the
outstanding shares of Common Stock (by reclassification or otherwise than by
payment of a dividend in shares of Common Stock) into a greater or lesser number
of shares of Common Stock, then in each such case the aggregate amount to which
holders of shares of Series B Preferred Stock were entitled immediately prior to
such event under the proviso in clause (1) of the preceding sentence shall be
adjusted by multiplying such amount by a fraction the numerator of which is the
number of shares of Common Stock outstanding immediately after such event and
the denominator of which is the number of shares of Common Stock that were
outstanding immediately prior to such event.

          Section 7.  Consolidation, Merger, etc.  In case the Corporation shall
                      ---------------------------
enter into any consolidation, merger, combination or other transaction in which
the shares of Common Stock are exchanged for or changed into other stock or
securities, cash and/or any other property, then in any such case each share of
Series B Preferred Stock shall at the same time be similarly exchanged or
changed into an amount per share, subject to the provision for adjustment
hereinafter set forth, equal to 100 times the aggregate amount of stock,
securities, cash and/or any other property (payable in kind), as the case may
be, into which or for which each share of

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<PAGE>

Common Stock is changed or exchanged. In the event the Corporation shall at any
time declare or pay any dividend on the Common Stock payable in shares of Common
Stock, or effect a subdivision or combination or consolidation of the
outstanding shares of Common Stock (by reclassification or otherwise than by
payment of a dividend in shares of Common Stock) into a greater or lesser number
of shares of Common Stock, then in each such case the amount set forth in the
preceding sentence with respect to the exchange or change of shares of Series B
Preferred Stock shall be adjusted by multiplying such amount by a fraction, the
numerator of which is the number of shares of Common Stock outstanding
immediately after such event and the denominator of which is the number of
shares of Common Stock that were outstanding immediately prior to such event.

          Section 8.  No Redemption.  The shares of Series B Preferred Stock
                      -------------
shall not be redeemable.

          Section 9.  Rank.  The Series B Preferred Stock shall rank, with
                      ----
respect to the payment of dividends and the distribution of assets, junior to
all series of any other class of the Corporation's Preferred Stock.

          Section 10.  Amendment.  The Restated Certificate of Incorporation of
                       ---------
the Corporation shall not be amended in any manner which would materially alter
or change the powers, preferences or special rights of the Series B Preferred
Stock so as to affect them adversely without the affirmative vote  of the
holders of at least two-thirds of the outstanding shares of Series B Preferred
Stock, voting together as a single class.


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