EXHIBIT 10.51

                            SECURED INSTALLMENT NOTE



$125,000,000.00                                            Cupertino, California
                                                                   July 26, 2005

     FOR VALUE  RECEIVED,  the  undersigned,  MISSION WEST  PROPERTIES,  L.P., a
Delaware limited  partnership,  and MISSION WEST PROPERTIES,  L.P. I, a Delaware
limited  partnership  doing business in California as MISSION WEST PROPERTIES I,
L.P., a Delaware limited partnership (collectively, "Borrower"), as maker, whose
address is 10050 Bandley Drive, Cupertino, California 95014, does hereby promise
to pay to the order of  ALLIANZ  LIFE  INSURANCE  COMPANY  OF NORTH  AMERICA,  a
Minnesota  corporation  ("Lender"),  as payee,  at its  office  c/o  Allianz  of
America,  Inc.,  55 Greens  Farms Road,  P.O.  Box 5160,  Westport,  Connecticut
06881-5160,  Attn:  Real  Estate  Department,  or such other place as Lender may
designate  in writing,  in lawful  money of the United  States of  America,  the
principal sum of ONE HUNDRED TWENTY-FIVE MILLION DOLLARS  ($125,000,000.00),  or
so much thereof as may be advanced, together with interest thereon to accrue, at
the rate of five and 22/100 percent (5.22%) per annum (the "Note Rate"), subject
to adjustment as herein provided.

     1. PAYMENT.  Said principal sum, and interest as herein  provided to accrue
on the unpaid principal, shall be paid as follows:

          5.  Interest only at the Note Rate from and including the date of this
     Note through and including  August 9, 2005, shall be paid in advance on the
     date of this Note.

          6. On each "Payment Date" to and including July 10, 2025,  payments of
     principal  and  interest  in the  amount  of  $840,212.00  shall be due and
     payable.  "Payment  Date"  means the tenth  (10th) day of each  consecutive
     calendar month for the term of this Note commencing September 10, 2005. The
     payments  due  under  this  subparagraph  (b) are each  called  a  "Monthly
     Installment."

          7. The entire remaining  principal  amount,  together with any accrued
     and unpaid interest (the "Final Installment"),  shall be due and payable in
     full on August 10, 2025 (the "Maturity Date").

          8.  Interest  shall be computed on the basis of a three  hundred sixty
     (360) day year consisting of twelve (12) months of thirty (30) days each.


     2. APPLICATION OF PAYMENTS. All payments shall be applied first to "Costs",
as defined below, and interest  accrued thereon;  then to the payment of escrows
for taxes and insurance,  if any; then to "Late Charges",  as defined below, and
interest accrued thereon,  if any; then to accrued and unpaid interest,  and the
remainder to the reduction of the  principal  balance  outstanding  from time to
time.  The term "Costs" shall mean any sums advanced by Lender as provided under
the terms of the "Deed of Trust," as defined below, and as provided in Paragraph
9 of this Note.

     3. LATE  CHARGE.  If any payment of  principal  or  interest,  or both,  so
provided for herein is not paid when due, it would be impracticable or extremely
difficult  to fix  the  actual  damages  resulting  therefrom  to  Lender,  and,
therefore,  Borrower  hereby agrees to pay to Lender a late charge of FIVE CENTS
($.05) for each ONE DOLLAR  ($1.00) so  overdue,  not as a penalty,  but for the
purpose of defraying the expenses  incident to handling such delinquent  payment
(a "Late Charge"). Such Late Charge represents the reasonable estimate of a fair
average  compensation  for the loss that may be  sustained  by Lender due to the
failure of Borrower to make timely payments.  Such Late Charge (i) shall be paid
without  prejudice to the right of Lender to collect any other amounts  provided
to be paid or to declare a default  under  this Note or the Deed of Trust,  (ii)
shall be  payable  not later  than the due date of the next  payment,  and (iii)
shall be secured by the "Security  Documents",  as defined  below.  In addition,
during the period of the  occurrence  of an "Event of  Default"  (as  defined in
Article IV of the Deed of Trust), the entire unpaid principal of this Note shall
accrue  interest at the Note Rate,  increased by FIVE HUNDRED (500) BASIS POINTS
(the "Default Rate").
                                                Initials: CEB
                                                         -----

     4.  PREPAYMENT.  (a) Subject to Paragraph 4(e), this Note may be prepaid in
full at any time subject to a prepayment  premium that may be substantial.  Such
premium  represents  consideration  to Lender for loss of yield and reinvestment
cost. The prepayment premium shall be determined by Lender and shall be equal to
the greater of (i) the  prepayment  premium  calculated in the manner  described
below,  or (ii) zero.  The  prepayment  premium  shall be an amount equal to the
excess,  if any,  between (x) the present value at the time of prepayment of the
remaining  scheduled  Monthly  Installments and the present value at the time of
prepayment of the Final  Installment,  both discounted on a monthly basis at the
"Index Rate," as defined  below,  and (y) the unpaid  principal  balance of this
Note at the time of prepayment. The "Index Rate" is defined as the current yield
at the time of prepayment of the Treasury Constant

<PAGE>

Maturity  (the  "TCM")  referenced  in the weekly  Federal  Reserve  Statistical
Release H-15 (519) for the week immediately  preceding the date on which written
request  for  prepayment  is received by Lender for the  maturity  most  closely
corresponding  to the  remaining  loan  term as  appropriately  interpolated  by
Lender, increased by TWENTY-FIVE (25) BASIS POINTS.

     (b) If the TCM ceases to be published  during the loan term, the Index Rate
shall be the average of the yield,  for the five (5) business days preceding the
date of  prepayment  of this  Note,  of the US  Treasury  Note or Bond  having a
remaining  term to maturity  and coupon rate most closely  corresponding  to the
remaining  term to maturity and interest rate of this Note.  The Index Rate will
apply for any  prepayment  made within  thirty  (30) days after such  request is
received by Lender, after which a more recent Index Rate may be used at the sole
discretion of Lender.

     (c) The prepayment  premium is to compensate Lender, and its successors and
assigns,  for the loss of  interest  it would  otherwise  earn on the  principal
hereof, if such principal were allowed to remain  outstanding,  and for the cost
incurred in connection with reinvestment of principal so prepaid,  at an earlier
date than the Maturity Date.

     (d) In the event of the  occurrence  of an Event of Default,  and following
acceleration  of the maturity of this Note, any payment of the amount  necessary
to satisfy this Note shall be deemed to be a voluntary  prepayment  of this Note
and shall be accompanied by the prepayment premium.

     (e) To the  extent  permitted  by law,  said  prepayment  premium  shall be
payable  regardless  of  whether  the loan  evidenced  by this  Note is  prepaid
voluntarily  or  involuntarily;  provided,  however,  that no  premium  shall be
payable on  prepayments  by  application  of the proceeds of any  proceedings in
eminent domain,  or proceedings in lieu thereof,  by application of the proceeds
of fire or other casualty insurance,  or by operation of Section 1.6 of the Deed
of  Trust;  and  provided,  further,  that no  premium  shall  be  payable  on a
prepayment  made during the ninety (90) days  immediately  prior to the Maturity
Date.

     (f) Borrower hereby waives all rights under  California  Civil Code Section
2954.10 which provides, in part, as follows:

     AN OBLIGEE WHICH ACCELERATES THE MATURITY DATE OF THE PRINCIPAL AND ACCRUED
     INTEREST,  PURSUANT TO CONTRACT,  ON ANY LOAN SECURED BY A MORTGAGE OR DEED
     OF TRUST ON REAL  PROPERTY  UPON THE  CONVEYANCE  OF ANY RIGHT,  TITLE,  OR
     INTEREST IN THAT  PROPERTY,  MAY NOT CLAIM,  EXACT,  OR COLLECT ANY CHARGE,
     FEE, OR PENALTY FOR ANY PREPAYMENT RESULTING FROM THAT ACCELERATION.

Borrower  understands and acknowledges  that Lender bargained for this waiver as
part of the  consideration  which induced Lender to enter into this transaction.
Borrower  initials  this  subparagraph  (f) for the  purpose of  evidencing  its
understanding of Civil Code Section 2954.10 recited in part above and Borrower's
agreement to the waiver of its terms.

                                                Initials: CEB
                                                         -----

     5.  SECURITY;  DEED OF TRUST.  (a) This Note is secured by a Deed of Trust,
Security Agreement,  Fixture Filing with Absolute Assignment of Rents (the "Deed
of  Trust"),   an  Absolute   Assignment  of  Leases,   Rents  and  Income  (the
"Assignment"), each of even date herewith, encumbering certain improved real and
personal  property located in Santa Clara County,  California,  and described in
the Deed of Trust (the "Property"),  and any other instruments, now or hereafter
executed  by  Borrower  in  favor  of  Lender,  which  evidence,  or  constitute
additional security for, this Note (as used herein the term "Security Documents"
mean, collectively, the Deed of Trust, the Assignment, and all other instruments
evidencing or securing this Note).  Reference to the Security  Documents is made
for all particulars including, without limitation, the obligation of Borrower to
pay monthly the "Reserves" described in Section 1.2 of the Deed of Trust.

     (b) This Note is given for an actual loan in the original  principal amount
of this Note and is the Note  referred  to in and  secured by the Deed of Trust.
All of the agreements,  conditions and covenants  contained in the Deed of Trust
which are to be kept and  performed  by Borrower  are hereby made a part of this
Note to the same extent and with the same force and effect as if they were fully
set forth herein, and Borrower covenants and agrees to keep and perform them, or
cause them to be kept and performed strictly in accordance with their terms.


     6. TRANSFER. (a) Except as otherwise specifically provided herein or in the
Deed of Trust, Borrower and all endorsers,  guarantors,  sureties, accommodation
parties  hereof and all other persons  liable or to become liable for all or any
part of this  indebtedness  agree that if the  Property  or any part  thereof or
interest  therein  is  sold,  assigned,   transferred,   conveyed,   encumbered,
hypothecated,  mortgaged or otherwise alienated by Borrower (excepting leases in
the ordinary course of managing income property,  transfers of personal property
authorized  herein  and  purchase  money  encumbrances  of  personalty)  whether
voluntarily,  involuntarily  or

<PAGE>

by operation of law,  without the prior written consent of Lender,  then Lender,
at its option, may declare the Note to be forthwith due and payable.

     (b) In the event of a transfer of title without the prior written  approval
of Lender,  Lender may, without notice to Borrower,  waive such default and deal
with such successor or successors in interest in the Property in the same manner
as  with  Borrower,  without  in any way  releasing,  discharging  or  otherwise
affecting  the  liability  of Borrower,  the  endorsers,  guarantors,  sureties,
accommodation  parties  hereof and all other persons  liable or to become liable
for  all or any  part  of  this  indebtedness.  No  sale  of  the  Property,  no
forbearance  on the part of Lender,  no extension of the time for the payment of
the  indebtedness  or any  change  in the  terms  of this  Note or the  Security
Documents  consented  to by  Lender  and  Borrower  shall in any way  whatsoever
operate to release,  discharge,  modify, change or affect the original liability
of Borrower and all such persons  liable or to become liable for all or any part
of this indebtedness, either in whole or in part.

     (c) Borrower shall not  voluntarily,  involuntarily  or by operation of law
sell, assign,  transfer or otherwise dispose of the collateral identified in the
Security  Documents  (the  "Collateral")  or any interest  therein and shall not
otherwise  do or  permit  anything  to be done or  occur  that  may  impair  the
Collateral as security under the Security  Documents  except so long as no Event
of Default exists under the Security  Documents,  Borrower shall be permitted to
sell or otherwise  dispose of the Collateral when  inadequate,  unserviceable or
unnecessary  for use in the  operation  of the Property or in the conduct of the
business of Borrower, upon replacing the same or substituting for the same other
Collateral  at least equal in value to the initial value of that disposed of and
in such a manner  so that  said  Collateral  shall be  subject  to the  security
interest created by the Security  Documents and so that the security interest of
Lender shall be the first priority security interest in said Collateral.  In the
event  the  Collateral  is sold in  connection  with the  sale of the  Property,
Borrower shall require,  as a condition of the sale, that the buyer specifically
agree to assume (preserving the exculpation  provisions  appearing at the end of
this Note)  Borrower's  obligations as to the security  interest  granted by the
Security  Documents and to execute  whatever  agreements  and filings are deemed
necessary  by  Lender  to  maintain  its  perfected  security  interest  in  the
Collateral.

     7.  DEFAULT;  REMEDIES.  (a) Should an "Event of  Default"  (as  defined in
Article IV of the Deed of Trust) occur,  then, or at any time  thereafter in the
manner authorized by law, the entire principal of this Note, irrespective of the
Maturity Date specified herein,  together with the then accrued interest thereon
shall, at the election of Lender hereof,  and without notice (except as required
by law) of such election, become immediately due and payable.

     (b) The rights or  remedies  of Lender,  as  provided  in this Note and the
Security  Documents,  shall be  cumulative  and  concurrent,  and may be pursued
singly,  successively or together against the property  described in the Deed of
Trust, and any other funds,  property or security held by Lender for the payment
hereof, at the sole discretion of Lender. The failure to exercise any such right
or remedy  shall in no event be  construed as a waiver or release of said rights
or remedies or of the right to exercise them at any later time.

     8.  WAIVER OF  NOTICE.  All  Borrowers,  endorsers,  guarantors,  sureties,
accommodation  parties  hereof and all other persons  liable or to become liable
for all or any part of this  indebtedness,  without  affecting their  liability,
waive diligence, presentment, protest and demand, and also notice of protest, of
demand,  of  nonpayment,  of dishonor and of maturity,  and all other notices of
every kind, other than any notice  specifically  required hereby, by the Deed of
Trust or other  Security  Documents,  and hereby consent to any extension of the
time of payment hereof, any and all renewals, extensions or modifications,  with
the consent of Borrower,  of the terms hereof, any release of all or any part of
the security given for the payment hereof, any acceptance of additional security
of any kind,  and any  release  of or resort to any  party  liable  for  payment
hereof.  Any such  renewals,  extensions  or  modifications  may be made without
notice to any of said parties,  and without  discharging said party's  liability
hereunder.

     9. COSTS. Borrower and all endorsers,  guarantors,  sureties, accommodation
parties  hereof and all other persons  liable or to become liable for all or any
part of  this  indebtedness  agree  to pay all  out-of-pocket  costs,  including
attorneys' and paralegal  fees, and costs,  and all costs of collection and suit
in the event the unpaid principal sum of this Note or any payment of interest or
principal and interest thereon, is not paid when due, or in the event it becomes
necessary  to  protect  the  security  for the  indebtedness  evidenced  hereby,
including,  without limitation,  any action arising out of the matters described
in Paragraph  15(b) below, or for the foreclosure by Lender of the Deed of Trust
or  other  Security  Documents,  or in the  event  Lender  is made  party to any
litigation solely because of the existence of the indebtedness evidenced by this
Note, or solely  because of the existence of the Deed of Trust or other Security
Documents,  whether  suit be  brought  or not,  and  whether  through  courts of
original  jurisdiction,  as well as in  courts  of  appellate  jurisdiction,  or
through a bankruptcy court or other legal proceedings.

     10. AMENDMENT. This Note may not be amended, modified or changed, nor shall
any waiver of any provision hereof be effective, except only by an instrument in
writing signed by the party against whom  enforcement of any waiver,  amendment,
change, modification or discharge is sought.

     11.  NOTICES.  Any notice  required or permitted  hereunder shall be deemed
sufficient if given in accordance with the provisions of the Deed of Trust.

     12. NO WAIVER BY  LENDER;  REMEDIES  CUMULATIVE.  (a)  Lender  shall not be
deemed,  by any act of omission or commission,  to have waived any of its rights
or remedies hereunder unless such waiver is in writing and signed by Lender, and
then only

<PAGE>

to the extent  specifically set forth in the writing. A waiver with reference to
one event shall not be construed as  continuing  or as a bar to or waiver of any
right or remedy as to a subsequent event.

     (b) The remedies of Lender as provided herein and in the Security Documents
shall be cumulative and concurrent and may be pursued  singularly,  successively
or together,  at the sole  discretion of Lender and may be exercised as often as
occasion  therefor  shall  occur;  and the failure to exercise any such right or
remedy shall in no event be construed as a waiver or release thereof.

     13. LIMITATION ON CHARGES. Notwithstanding any provisions in the Note or in
the Deed of Trust to the  contrary,  the total  liability  for  payments  in the
nature of interest,  including, but not limited to, prepayment premiums, default
interest  and late fees shall not  exceed the limits  imposed by the laws of the
State of  California  or the  United  States  of  America  relating  to  maximum
allowable  charges of interest,  if applicable.  Lender shall not be entitled to
receive,  collect or apply,  as interest on the  indebtedness  evidenced by this
Note, any amount in excess of the maximum  lawful rate of interest  permitted to
be charged by such laws,  if  applicable.  In the event  Lender  ever  receives,
collects  or applies as interest  any such  excess,  such amount  which would be
excessive  interest shall be applied to reduce the unpaid  principal  balance of
the indebtedness evidenced by this Note. If the unpaid principal balance of such
indebtedness  is paid in full,  any remaining  excess shall be forthwith paid to
Borrower.

     14. GOVERNING LAW. This Note is to be construed in accordance with the laws
of the State of  California.  In case any one or more of the  provisions of this
Note shall, for any reason, be held to be invalid,  illegal, or unenforceable in
any respect, such invalidity,  illegality,  or unenforceability shall not affect
any other  provisions of this Note,  and this Note shall be construed as if such
invalid, illegal, or unenforceable provision had never been contained herein. If
any one or more of the provisions contained in this Note shall for any reason be
held to be excessive as to amount, time, duration,  scope, activity, or subject,
such provisions shall be construed by limiting and reducing the provisions so as
to make such  provisions  enforceable  to the  extent  compatible  with the then
existing applicable law.

     15. LIMITED  LIABILITY.  (a) Except as set forth in Paragraph  15(b) below,
the liability of Borrower with respect to the payment of principal, interest and
Costs  hereunder  and with  respect to  performance  by Borrower  of  Borrower's
obligations and any and all other liability hereunder,  under the Deed of Trust,
and  under  the  other  Security   Documents,   shall  be  "non-recourse"   and,
accordingly, Lender's source of satisfaction of said indebtedness and Borrower's
other  obligations  hereunder and under the other  Security  Documents  shall be
limited to the Property and the rents, issues, and profits from the Property and
Lender shall not seek to procure payment out of any other assets of Borrower, or
any person or entity  comprising  Borrower,  nor to seek  judgment  for any sums
which are or may be payable  under this Note or under any of the other  Security
Documents,  as well as any claim or judgment (except as hereafter  provided) for
any deficiency remaining after foreclosure of the Deed of Trust. Notwithstanding
the  above,  nothing  herein  contained  shall  be  deemed  to be a  release  or
impairment of the indebtedness  evidenced by this Note or the security  therefor
intended  by the  Security  Documents,  or be deemed  to  preclude  Lender  from
exercising  its rights to  foreclose  the Deed of Trust or to enforce any of its
other rights or remedies under the Security Documents, subject to the provisions
of this Paragraph 15.

     (b)  Notwithstanding the foregoing,  it is expressly  understood and agreed
that the aforesaid  limitation  on liability  shall in no way affect or apply to
Borrower's  continued  personal liability for, and Lender's right to recover the
following:

          (1) The  retention  by  Borrower  of any  rental  income or other cash
     collected  arising with  respect to the  Property,  which was  collected by
     Borrower after an Event of Default has occurred under the Deed of Trust and
     while such Event of Default was or is continuing;

          (2) The retention by Borrower of any prepaid rental  income,  deposit,
     or other  prepaid  income  under any  lease of all or part of the  Property
     which was unearned as of the  occurrence  of the Event of Default under the
     Deed of Trust;

          (3) The replacement cost of any Property,  including personal property
     or fixtures  owned by  Borrower,  encumbered  by the Deed of Trust which is
     damaged,  destroyed,  removed, or disposed of and not repaired,  rebuilt or
     replaced  as  required  by the  Deed  of  Trust,  to the  extent  that  the
     replacement cost of such Property  exceeds the insurance  proceeds (if any)
     received by Borrower and/or Lender as a result of such events;

          (4) The  misapplication  of any proceeds,  to the extent of misapplied
     proceeds, under any insurance policies pertaining to the Property or awards
     resulting from  condemnation or the exercise of the power of eminent domain
     or by reason of damage or destruction to any portion of the Property or any
     building or buildings located thereon;

          (5) Any unpaid real estate taxes,  utilities,  assessments,  insurance
     premiums and any other expenses (excluding  principal and interest payments
     under the Loan) relating to the Property which were received by Borrower or
     which  accrued  prior to the date that  Lender  acquired  ownership  of the
     Property, less a credit for any unapplied Reserves;

          (6) Damages and losses suffered or incurred by Lender as the result of
     bad faith, waste, fraud or material misrepresentation by Borrower under the
     Security  Documents  or  Borrower's  loan  application,   or  arising  from

<PAGE>

     Borrower's  bankruptcy,  or arising  from  Borrower's  default or liability
     under any leases of the Property in effect  during the period of Borrower's
     ownership  of the  Property,  or arising from the  termination  of any such
     leases as a result of a default by Borrower as lessor thereunder, or as the
     result of any  prohibited  transfer of title to the  Property or  ownership
     interest in Borrower;

          (7) All  indemnification  obligations  by  Borrower in favor of Lender
     under the Environmental  Indemnity Agreement of even date herewith given to
     Lender  by  Borrower  and  Mission  West   Properties,   Inc.,  a  Maryland
     corporation ("Principal of Borrower"); and

          (8) The out-of-pocket costs and expenses incurred by Lender to enforce
     Lender's rights under this Paragraph 15(b),  including  attorneys' fees and
     costs (including the costs of paralegals).

     (c)  Notwithstanding  the foregoing,  the agreement of Lender not to pursue
recourse  liability against Borrower as set forth in Paragraph 15(a) above SHALL
BECOME  NULL AND  VOID  and  shall be of no  further  force or  effect,  and the
indebtedness of Borrower under this Note and all other sums due from Borrower to
Lender under the Loan shall immediately become FULLY RECOURSE to Borrower and to
Principal  of  Borrower  and  any  other  guarantor  of the  Loan,  jointly  and
severally, in the event of:

          (a) A default  by  Borrower  or any member of  Borrower  of any of the
     prohibitions of Section 1.11 of the Deed of Trust; or

          (b) A voluntary  bankruptcy or  insolvency  proceeding is commenced by
     Borrower  under the U.S.  Bankruptcy  Code or any similar  federal or state
     law; or

          (c) An  involuntary  bankruptcy or insolvency  proceeding is commenced
     against  Borrower or Principal of Borrower  (other than by Lender) and such
     action or  proceeding  is not  dismissed  within ninety (90) days after the
     date of filing hereof.

     (d) Nothing  herein  shall be deemed a waiver of any right which Lender may
have under any provision of the U.S. Bankruptcy Code (i) to file a claim for the
full amount of the  indebtedness  of  Borrower to Lender  under this Note or the
Security  Documents,  or (ii) to require that all  collateral  encumbered by the
Deed of Trust shall continue to secure all  indebtedness  of Borrower under this
Note and the Security Documents in accordance with the terms thereof.

     (16)  BROKER.  This  Note is made in  connection  with a loan  arranged  by
Northmarq  Capital,  San  Francisco,  California,  a licensed real estate broker
under the Laws of California.

     (17)  SUCCESSORS.  Whenever used herein,  the singular number shall include
the plural, the plural the singular, and the words "Borrower" and "Lender" shall
be deemed to include their successors and assigns.

     (18)  RECEIPT  OF  DOCUMENTS.   By  signing  below,  Borrower  acknowledges
receiving a copy of this Note and acknowledges receiving a copy of all documents
signed by Borrower in connection with this Note.




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<PAGE>


     IN WITNESS  WHEREOF,  Borrower has executed  this Note as of the date first
above written.

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

                                            By:   Mission West Properties, Inc.,
                                                  a Maryland corporation,
                                                  General Partner


                                                  By: /s/ Carl E. Berg

                                                  Name: Carl E. Berg

                                                  Title: Chief Executive Officer


                                            MISSION WEST PROPERTIES, L.P. I,
                                            a Delaware limited partnership doing
                                            business in California as
                                            MISSION WEST PROPERTIES I, L.P.,
                                            a Delaware limited partnership

                                            By:   Mission West Properties, Inc.,
                                                  a Maryland corporation,
                                                  General Partner


                                                  By: /s/ Carl E. Berg

                                                  Name: Carl E. Berg

                                                  Title: Chief Executive Officer