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                                                                   Exhibit 10.18

                   ELEVENTH AMENDMENT TO AMENDED AND RESTATED
                        AGREEMENT OF LIMITED PARTNERSHIP
                                       OF
                     BRANDYWINE OPERATING PARTNERSHIP, L.P.

                  THIS ELEVENTH AMENDMENT, dated December 30, 2003 (this
"Amendment"), amends and supplements the Amended and Restated Agreement of
Limited Partnership Agreement (as heretofore amended and supplemented to date,
the "Partnership Agreement") of BRANDYWINE OPERATING PARTNERSHIP, L.P., a
Delaware limited partnership (the "Partnership"). Capitalized terms used herein
but not defined herein shall have the meanings given to such terms in the
Partnership Agreement.

                                   BACKGROUND
                                   ----------

         G. Pursuant to the Partnership Agreement, Brandywine Realty Trust (the
"General Partner"), as the general partner of the Partnership, has the power and
authority to issue additional Partnership Interests and Units in one or more
newly created classes of Partnership Interests to persons on such terms and
conditions as the General Partner may deem appropriate.

         H. The General Partner, pursuant to the exercise of such power and
authority and in accordance with the Partnership Agreement, has determined to
execute this Amendment to the Partnership Agreement to create a new class of
Partnership Interests designated as the Series D Preferred Mirror Units having
designations, preferences and other rights which are substantially the same as
the economic rights of the 7.50% Series C Senior Cumulative Redeemable Preferred
Shares of Beneficial Interest of the General Partner (the "Series C Preferred
Shares") and to evidence the issuance of such additional Partnership Interests
to the General Partner in exchange for the General Partner's contribution to the
Partnership of the net proceeds of the sale of the Series C Preferred Shares.

                  NOW, THEREFORE, in consideration of the mutual covenants and
agreements herein contained and other good and valuable consideration, the
receipt, adequacy and sufficiency of which are hereby acknowledged, the parties
hereto, intending to be legally bound, hereby amend the Partnership Agreement as
follows:

                  1. In accordance with the Partnership Agreement, the
Partnership Agreement is hereby amended to establish, and to issue to the
General Partner, the Series D Preferred Mirror Units having the designations,
preferences and other rights set forth below:

                        (i)    Designation and Number. A class of Partnership
                               Interests designated as Series D Preferred Mirror
                               Units is hereby established. The number of Series
                               D Preferred Mirror Units shall be 4,600,000. The
                               stated value of each Series D Preferred Mirror
                               Unit shall be $25.00 (the "Stated Value").






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                        (ii)   Rank. The Series D Preferred Mirror Units will,
                               with respect to distribution rights and rights
                               upon liquidation, dissolution or winding up of
                               the Partnership, rank (a) senior to the Class A
                               Units and all Partnership Interests ranking
                               junior to the Series D Preferred Mirror Units;
                               (b) on a parity with the Partnership Interests
                               designated as Series A Preferred Mirror Units and
                               the Partnership Interests designated as Series B
                               Preferred Units and all Partnership Interests
                               issued by the Partnership after the date of this
                               Amendment the terms of which specifically provide
                               that such Partnership Interests rank on a parity
                               with the Series D Preferred Mirror Units; and (c)
                               junior to all Partnership Interests issued by the
                               Partnership the terms of which specifically
                               provide that such Partnership Interests rank
                               senior to the Series D Preferred Mirror Units.

                        (iii)  Distributions.

                               (A)    Pursuant to Section 6.1 of the Partnership
                                      Agreement, holders of Series D Preferred
                                      Mirror Units shall be entitled to receive,
                                      out of funds legally available therefor,
                                      cumulative quarterly cash distributions
                                      equal to the amount of the cumulative
                                      quarterly cash distributions payable on
                                      the Series C Preferred Shares. Such
                                      distributions shall be payable quarterly
                                      in arrears on or before the date on which
                                      distributions on the Series C Preferred
                                      Shares are payable (each a "Series D
                                      Preferred Mirror Unit Distribution Payment
                                      Date").

                               (B)    No distributions on Series D Preferred
                                      Mirror Units shall be authorized or paid
                                      or set apart for payment by the
                                      Partnership at such time as the terms and
                                      provisions of any agreement of the
                                      Partnership, including any agreement
                                      relating to its indebtedness, prohibits
                                      such authorization, payment or setting
                                      apart for payment or provides that such
                                      authorization, payment or setting apart
                                      for payment would constitute a breach
                                      thereof, or a default thereunder, or if
                                      such authorization or payment shall be
                                      restricted or prohibited by law.

                               (C)    Notwithstanding the foregoing,
                                      distributions with respect to the Series D
                                      Preferred Mirror Units will accrue whether
                                      or not the terms and provisions set forth
                                      in Section 1(c)(ii) at any time prohibit
                                      the current payment of distributions,
                                      whether or not there are funds legally
                                      available for such distributions and
                                      whether or not such distributions are
                                      authorized. Accrued but unpaid
                                      distributions on the Series D Preferred
                                      Mirror Units will accumulate as of the
                                      Series D Preferred Mirror Unit
                                      Distribution Payment Date on which they
                                      first become payable.



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                               (D)    When distributions are not paid in full
                                      (or a sum sufficient for such full payment
                                      is not so set apart) upon the Series D
                                      Preferred Mirror Units and any other
                                      Partnership Interests ranking on a parity
                                      as to distributions with the Series D
                                      Preferred Mirror Units, including the
                                      Series A Preferred Mirror Units and the
                                      Series B Preferred Units, all
                                      distributions authorized upon the Series D
                                      Preferred Mirror Units and any other
                                      Partnership Interests ranking on a parity
                                      as to distributions with the Series D
                                      Preferred Mirror Units shall be authorized
                                      pro rata so that the amount of
                                      distributions authorized per Partnership
                                      Unit of Series D Preferred Mirror Units
                                      and such other Partnership Interests shall
                                      in all cases bear to each other the same
                                      ratio that accrued distributions per
                                      Partnership Unit on the Series D Preferred
                                      Mirror Units and such other Partnership
                                      Interests (which shall not, with respect
                                      to such other Partnership Interests,
                                      include any accrual in respect of unpaid
                                      distributions for prior distribution
                                      periods if such other Partnership
                                      Interests do not have a cumulative
                                      distribution) bear to each other. Any
                                      distribution payment or payments on Series
                                      D Preferred Mirror Units which may be in
                                      arrears shall accrue distributions at the
                                      rate of 7.50% per annum.

                               (E)    Except as provided in Section 1(c)(iv),
                                      unless full cumulative distributions on
                                      the Series D Preferred Mirror Units have
                                      been or contemporaneously are authorized
                                      and paid or authorized and a sum
                                      sufficient for the payment thereof is set
                                      apart for payment for all past
                                      distribution periods and the then current
                                      distribution period, no distributions
                                      (other than in Partnership Interests
                                      ranking junior to the Series D Preferred
                                      Mirror Units as to distributions and upon
                                      liquidation, dissolution or winding up)
                                      shall be authorized or paid or set aside
                                      for payment nor shall any other
                                      distribution be authorized or made upon
                                      the Class A Units, the Series A Preferred
                                      Mirror Units, the Series B Preferred Units
                                      or any other Partnership Interests ranking
                                      junior to or on a parity with the Series D
                                      Preferred Mirror Units as to distributions
                                      or upon liquidation, dissolution or
                                      winding up, nor shall any Class A Units,
                                      Series A Preferred Mirror Units, Series B
                                      Preferred Units or any other Partnership
                                      Interests ranking junior to or on a parity
                                      with the Series C Preferred Shares as to
                                      distributions or upon liquidation be
                                      redeemed, purchased or otherwise acquired
                                      for any consideration (or any moneys be
                                      paid to or made available for a sinking
                                      fund for the redemption of any such units
                                      or other Partnership Interests) by the
                                      Partnership or any other entity controlled
                                      directly or indirectly by the Partnership
                                      (except by conversion into or exchange for
                                      Partnership Interests ranking junior to
                                      the Series D Preferred Mirror Units as to
                                      distributions and upon liquidation,
                                      dissolution or winding up).



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                               (F)    Holders of the Series D Preferred Mirror
                                      Units shall not be entitled to any
                                      distribution, whether payable in cash,
                                      property or Partnership Units in excess of
                                      full cumulative distributions on the
                                      Series D Preferred Mirror Units as
                                      described above. Any distribution payment
                                      made on the Series D Preferred Mirror
                                      Units shall first be credited against the
                                      earliest accrued but unpaid distribution
                                      due with respect to such Series D
                                      Preferred Mirror Units which remains
                                      payable.

                        (iv)   Liquidation Preference.

                               (A)    Upon any voluntary or involuntary
                                      liquidation, dissolution or winding up of
                                      the affairs of the Partnership, the
                                      holders of Series D Preferred Mirror Units
                                      then outstanding are entitled to be paid
                                      out of the assets of the Partnership
                                      available for distribution to the Partners
                                      pursuant to Section 13.5(a) of the
                                      Partnership Agreement a liquidation
                                      preference equal to the Stated Value per
                                      Series D Preferred Mirror Unit, plus an
                                      amount equal to any accrued and unpaid
                                      distributions to the date of payment,
                                      before any distribution of assets is made
                                      to holders of Class A Units and GP Units
                                      or any other Partnership Interests that
                                      rank junior to the Series D Preferred
                                      Mirror Units upon liquidation, dissolution
                                      or winding up.

                               (B)    In the event that, upon any such voluntary
                                      or involuntary liquidation, dissolution or
                                      winding up, the available assets of the
                                      Partnership are insufficient to pay the
                                      amount of the liquidating distributions on
                                      all outstanding Series D Preferred Mirror
                                      Units and the corresponding amounts
                                      payable on all other Partnership Interests
                                      ranking on a parity with the Series D
                                      Preferred Mirror Units in the distribution
                                      of assets, including the Series A
                                      Preferred Mirror Units and the Series B
                                      Preferred Units, then such assets shall be
                                      allocated among the Series D Preferred
                                      Mirror Units, as a class, and each class
                                      or series of such other such Partnership
                                      Interests, as classes, in proportion to
                                      the full liquidating distributions to
                                      which they would otherwise be respectively
                                      entitled.




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                               (C)    After payment of the full amount of the
                                      liquidating distributions to which they
                                      are entitled, the holders of Series D
                                      Preferred Mirror Units will have no right
                                      or claim to any of the remaining assets of
                                      the Partnership.

                               (D)    The consolidation or merger of the
                                      Partnership with or into any other
                                      partnership, limited liability company,
                                      corporation, trust or entity or of any
                                      other partnership, limited liability
                                      company, corporation, trust or other
                                      entity with or into the Partnership, or
                                      the sale, lease or conveyance of all or
                                      substantially all of the property or
                                      business of the Partnership, shall not be
                                      deemed to constitute a liquidation,
                                      dissolution or winding up of the
                                      Partnership for purposes of this Section
                                      1(d).

                        (v)    Redemption. In connection with a redemption by
                               the General Partner of any or all of the Series C
                               Preferred Shares, the Partnership shall provide
                               cash to the General Partner for such purpose
                               which shall be equal to the redemption price
                               (including accrued and unpaid distributions) of
                               the Series C Preferred Shares to be redeemed and
                               in exchange one Series D Preferred Mirror Unit
                               shall be canceled with respect to each Series C
                               Preferred Share so redeemed. From and after the
                               date on which the Series C Preferred Shares are
                               redeemed, the Series D Preferred Mirror Units so
                               canceled shall no longer be outstanding and all
                               rights hereunder, to distributions or otherwise,
                               with respect to such Series D Preferred Mirror
                               Units shall cease.

                        (vi)   Allocations. Allocations of the Partnership's
                               items of income, gain, loss and deduction shall
                               be allocated among holders of Series D Preferred
                               Mirror Units in accordance with Article VII of
                               the Partnership Agreement.

                  2. Subparagraph (g) of Section 7.2 of the Partnership
Agreement is amended and restated in its entirety as follows:

                        (g) Priority Allocation. All or a portion of the Net
Income of the Partnership for the Fiscal Year, if any, shall be specially
allocated to the Partners holding Series A Preferred Mirror Units, Series B
Preferred Units and Series D Preferred Mirror Units in proportion to the
cumulative distributions each has received pursuant to Sections 6.1, 6.2, and
13.5 hereof and, with respect to the Partners holding Series A Preferred Mirror
Units, Section 1(c) and 1(d) of the Fourth Amendment to this Agreement or, with
respect to Partners holding Series B Preferred Units, Section 1.C and 1.D of the
Fifth Amendment to this Agreement, or, with respect to the Partners holding
Series D Preferred Mirror Unit, Section 1(c) and 1(d) of the Eleventh Amendment
to this Agreement, from the commencement of the Partnership to the end of such
Fiscal Year, in an amount equal to the excess, if any, of the sum of (i) the
aggregate Net Loss allocated to such Partners pursuant to Section 7.1(b) hereof





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for all prior Fiscal Years, if any, and (ii) the aggregate distributions
received by such Partners pursuant to Sections 6.1, 6.2, and 13.5 hereof and,
with respect to Partners holding Series A Preferred Mirror Units, Section 1(c)
and 1(d) of the Fourth Amendment to this Agreement or, with respect to Partners
holding Series B Preferred Units, Section 1.C and 1.D of the Fifth Amendment to
this Agreement, or, with respect to Partners holding Series D Preferred Mirror
Units, Section 1(c) and 1(d) of the Eleventh Amendment to this Agreement from
the commencement of the Partnership to the end of such Fiscal Year, over the
aggregate items of Net Income allocated to such Partners pursuant to this
Section 7.2(g) for all prior Fiscal Years.

                  3. Except as expressly set forth in this Amendment to the
Partnership Agreement, the Partnership Agreement is hereby ratified and
confirmed in each and every respect.

                  IN WITNESS WHEREOF, this Eleventh Amendment to the Amended and
Restated Agreement of Limited Partnership of Brandywine Operating Partnership,
L.P. has been executed and delivered as of the date first above written.

                                  GENERAL PARTNER:

                                  BRANDYWINE REALTY TRUST

                                  By:_______________________________________
                                     Name: Gerard H. Sweeney
                                     Its:  President and Chief Executive Officer







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