EXHIBIT 10.33.1

Loan No. 332757

                       FIRST AMENDMENT TO PROMISSORY NOTE
                        (Mission West Properties, L.P. I)

     THIS FIRST AMENDMENT TO PROMISSORY NOTE (this "First Amendment") is made as
of the 5th day of November,  2008 between,  MISSION WEST  PROPERTIES,  L.P. I, a
Delaware limited partnership,  10050 Bandley Drive,  Cupertino, CA 95014, herein
called  "Borrower",  and THE  NORTHWESTERN  MUTUAL  LIFE  INSURANCE  COMPANY,  a
Wisconsin corporation,  720 East Wisconsin Avenue,  Milwaukee,  Wisconsin 53202,
hereinafter called "Northwestern".

                                    RECITALS

     WHEREAS, Northwestern is the owner of a certain Promissory Note executed by
MISSION WEST  PROPERTIES,  L.P. I on January 3, 2003 in the  original  principal
amount of  TWENTY-NINE  MILLION  EIGHT  HUNDRED  ELEVEN  THOUSAND  THREE HUNDRED
SIXTY-NINE  DOLLARS  ($29,811,369.00)  (the  "Note")  secured  by,  among  other
security,  i) a lien against certain property in the County of Santa Clara, City
of  Santa  Clara  and  the  City  of  Cupertino,  State  of  California  as more
particularly  described in that  certain  Deed of Trust and  Security  Agreement
(First Priority) dated January 3, 2003 executed by Borrower and recorded January
9,  2003 as  Document  No.  16735202  in the  records  of  Santa  Clara  County,
California,  as amended by that  certain  First  Amendment  to Deed of Trust and
Security  Agreement  (First  Priority) and First  Amendment to Deed of Trust and
Security Agreement (Second Priority) and First Amendment to Absolute  Assignment
of Leases and Rents (Mission West Properties,  L.P.) dated of even date herewith
between Lender and Borrower (the "Lien Instrument");  ii) a lien against certain
property in the County of Santa Clara,  City of San Jose, State of California as
more particularly described in that certain Deed of Trust and Security Agreement
and  Assignment  of Leases and Rents  (Second  Priority)  dated  January 3, 2003
executed by Mission West Properties, L.P. ("MW") and recorded January 9, 2003 as
Document  No.  16735207  in the records of Santa Clara  County,  California,  as
amended by that certain First Amendment to Deed of Trust and Security  Agreement
(First  Priority)  and First  Amendment to Deed of Trust and Security  Agreement
(Second Priority) and First Amendment to Absolute Assignment of Leases and Rents
(Mission West  Properties,  L.P.) dated of even date herewith between Lender and
MW (the "MW Second Lien  Instrument");  and iii) a lien against certain property
in the County of Santa Clara,  City of San Jose and the City of Milpitas,  State
of California as more  particularly  described in that certain Deed of Trust and
Security  Agreement and Assignment of Leases and Rents (Second  Priority)  dated
January 3, 2003  executed by Mission  West  Properties,  L.P.  II  ("MWII")  and
recorded  January 9, 2003 as Document No. 16735203 in the records of Santa Clara
County,  California, as amended by that certain First Amendment to Deed of Trust
and Security Agreement (First Priority) and First Amendment to Deed of Trust and
Security Agreement (Second Priority) and First Amendment to Absolute  Assignment
of Leases and Rents (Mission West Properties,  L.P.) dated of even date herewith
between Lender and MWII (the "MWII Second Lien Instrument");

     AND WHEREAS, Northwestern has agreed to amend certain terms of the Note.

     NOW, THEREFORE,  in consideration of the above and of the mutual agreements
herein contained, the undersigned parties agree to the following:

     1. Unless  otherwise  defined  herein or otherwise  indicated,  capitalized
words and terms used but not defined  herein shall have the meaning  ascribed to
them in the Note.

     2. The amount of the  Promissory  Note shall be  amended  from  TWENTY-NINE
MILLION  EIGHT  HUNDRED  ELEVEN  THOUSAND  THREE  HUNDRED   SIXTY-NINE   DOLLARS
($29,811,369.00)  TO  TWENTY-THREE  MILLION ONE HUNDRED  FORTY-TWO  THOUSAND SIX
HUNDRED THIRTY-ONE DOLLARS ($23,142,631.00).

     3. The monthly  payment  amount of principal and interest  shall be amended
from $207,433.00 to $197,276.00.

     4.  Schedule 1 attached  to the Note is deleted and  replaced  with the new
Schedule 1 attached hereto.

     5. This First Amendment may be executed in two or more  counterparts,  each
of which shall be deemed an  original,  and it shall not be  necessary in making
proof of this First  Amendment  to  produce  or  account  for more than one such
counterpart.  Separate  counterparts  of this First Amendment may be executed by
the parties  hereto to the same effect as if all the  parties had  executed  the
same counterpart.

     6. Except as hereby amended the Note shall remain in full force and effect,
unchanged and in all respects, ratified and confirmed.

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<PAGE>

     7.  Nothing  herein  contained  shall  affect  the  priority  of  the  Lien
Instrument,  the MW Second Lien  Instrument and the MWII Second Lien  Instrument
over other liens,  charges,  encumbrances or conveyances nor shall it release or
change the liability of any party who may now or hereafter be liable,  primarily
or secondarily, under or on account of the Note.

                                     - 2 -
<PAGE>



         IN WITNESS WHEREOF, this First Amendment has been executed by the
undersigned as of the date and year first above written.

THE NORTHWESTERN MUTUAL LIFE INSURANCE COMPANY,
                        a Wisconsin corporation

                        By:  Northwestern Investment Management Company, LLC,
                             a Delaware limited liability company, its
                             wholly-owned affiliate and authorized
                             representative

                             By:      /S/ Michael P. Cusick
                                ------------------------------------------------
                                Michael P. Cusick
                                Managing Director

                             Attest:  /S/ Donna L. Lemanczyk
                                    --------------------------------------------
                                    Donna L. Lemanczyk
                                    Assistant Secretary
(corporate seal)


                        MISSION WEST PROPERTIES, L.P.,
                        a Delaware limited partnership

                        By:  Mission West Properties, Inc.,
                             a Maryland corporation,
                             its general partner

                             By:      /S/ Raymond V. Marino
                                -----------------------------------------
                             Name:     Raymond V. Marino
                                 ----------------------------------------
                             Title:   President & COO
                                   --------------------------------------





                  (Consent of Guarantor and signature follows)

                                     - 3 -

<PAGE>




         CONSENT OF GUARANTOR:


The  undersigned  guarantor,  who is a party  under that  certain  Guarantee  of
Recourse  Obligations  dated as of June 3, 2003,  joins in the execution of this
document for the purpose of acknowledging  the above. The undersigned  guarantor
confirms that, except as herein modified,  the Guarantee of Recourse Obligations
remains in full force and effect,  unchanged  and in all  respects  ratified and
confirmed.

                                   GUARANTOR:

                                   MISSION WEST PROPERTIES, INC.,
                                   a Maryland corporation

                                   By:      /S/ Raymond V. Marino
                                      ----------------------------------------
                                   Name:    Raymond V. Marino
                                        --------------------------------------
                                   Title:   President & COO
                                         -------------------------------------

                                     - 4 -


<PAGE>




                                   SCHEDULE 1

                                Promissory Notes


Promissory Note dated January 3, 2003 executed by Mission West Properties,  L.P.
in the original  principal  amount of  $28,868,655.00,  as amended by that First
Amendment To Promissory Note dated November 5, 2008 now in the principal  amount
of $49,151,169.00.

Promissory Note dated January 3, 2003 executed by Mission West Properties,  L.P.
I in the original principal amount of  $29,811,369.00,  as amended by that First
Amendment To Promissory Note dated November 5, 2008 now in the principal  amount
of $23,228,942.00.

Promissory Note dated January 3, 2003 executed by Mission West Properties,  L.P.
II in the original principal amount of $41,319,976.00,  as amended by that First
Amendment To Promissory Note dated November 5, 2008 now in the principal  amount
of $9,551,366.00.

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