TWENTY FIRST AMENDMENT TO SECOND AMENDED AND RESTATED LIMITED PARTNERSHIP AGREEMENT OF CORPORATEOFFICE PROPERTIES, L.P.

Exhibit 10.1

TWENTY FIRST AMENDMENT TO

SECOND AMENDED AND RESTATED

LIMITED PARTNERSHIP AGREEMENT OF

CORPORATE OFFICE PROPERTIES, L.P.

THIS TWENTY FIRST AMENDMENT (the “Amendment”) to the Second Amended and Restated Limited Partnership Agreement of Corporate Office Properties, L.P., a Delaware limited partnership (the “Partnership”), is made and entered into as of July 20, 2006, by and among the undersigned parties.

Recitals

A.   The Partnership is a limited partnership organized under the Delaware Revised Uniform Limited Partnership Act (the “Act”) and governed by that certain Second Amended and Restated Limited Partnership Agreement dated as of January, 1999, as amended by that certain First Amendment to Second Amended and Restated Limited Partnership Agreement dated as of  December 21, 1999, that certain Second Amendment to Second Amended and Restated Limited Partnership Agreement dated as of December 21, 1999, that certain Third Amendment to Second Amended and Restated Limited Partnership Agreement dated as of  September 29, 2000, that certain Fourth Amendment to Second Amended and Restated Limited Partnership Agreement dated as of November 27, 2000, that certain Fifth Amendment to Second Amended and Restated Limited Partnership Agreement dated as of January 25, 2001, that certain Sixth Amendment to Second Amended and Restated Limited Partnership Agreement dated as of April 6, 2001, that certain Seventh Amendment to the Second Amended and Restated Partnership Agreement dated as of August 30, 2001, that certain Eighth Amendment to the Second Amended and Restated Partnership Agreement dated September 14, 2001, that certain Ninth Amendment to the Second Amended and Restated Partnership Agreement dated October 16, 2001, that certain Tenth Amendment to the Second Amended and Restated Partnership Agreement dated December 29, 2001, that certain Eleventh Amendment to the Second Amended and Restated Partnership Agreement dated December 15, 2002, that certain Twelfth Amendment to the Second Amended and Restated Partnership Agreement dated June 2, 2003, that certain Thirteenth Amendment to the Second Amended and Restated Partnership Agreement dated August 11, 2003, that certain Fourteenth Amendment to the Second Amended and Restated Partnership Agreement dated December 18, 2003, that certain Fifteenth Amendment to the Second Amended and Restated Partnership Agreement dated  January 31, 2004, that certain Sixteenth Amendment to the Second Amended and Restated Partnership Agreement dated April 15, 2004, that certain Seventeenth Amendment to the Second Amended and Restated Partnership Agreement dated September 23, 2004, that certain Eighteenth Amendment to the Second Amended and Restated Partnership Agreement dated April 18, 2005, that certain Nineteenth Amendment to the Second Amended and Restated Partnership Agreement dated July 8, 2005 and that certain Twentieth Amendment to the Second Amended and Restated Partnership Agreement dated June 29, 2006 (as amended, the “Agreement”).

B.   The sole general partner of the Partnership is Corporate Office Properties Trust, a real estate investment trust formed under the laws of the State of Maryland (the “General Partner”).

C.   The General Partner has issued 3,390,000 of its 7.625 % Series J Cumulative Redeemable Preferred Shares (the “Series J Preferred REIT Shares”) in a public offering (the “Offering”).

D.   As required under Sections 4.2(B) and (C) of the Agreement, the General Partner intends to transfer the net proceeds of the Offering (or cause them to be transferred) to or for the benefit of the Partnership in exchange for additional Partnership Interests in the Partnership having designations, rights and preferences substantially similar to the economic rights of the holders of the Series J Preferred REIT Shares (the “Series J Preferred Units”).

E.    The General Partner desires to amend the Agreement to acknowledge the contribution of the net proceeds of the Offering by the General Partner to the Partnership in exchange for the Series J Preferred Units.  Unless otherwise defined herein, all capitalized terms used in this Amendment shall have the same meanings as set forth in the Agreement.

NOW THEREFORE, in consideration of the foregoing and of the mutual premises set forth herein, the General Partner, intending to be legally bound hereby, hereby amends the Agreement as follows, effective as of the date set forth above.

1.    The foregoing recitals to this Amendment are hereby incorporated in and made a part of this Amendment.

(a)          Upon consummation of the Offering, the General Partner shall contribute the net proceeds of the Offering to the Partnership.

(b)         Upon the contribution of the net proceeds of the Offering to the Partnership by the General Partner, and in accordance with Section 4.2(B) of the Agreement, the Partnership shall issue to the General Partner 3,390,000 Series J Preferred Units, equal to the number of Series J Preferred REIT Shares issued by the General Partner in connection with the Offering.

(c)          For purposes of the Agreement, including the maintenance of Capital Accounts, the General Partner shall be treated as making a Capital Contribution of $82,080,375, equal to the product of $24.2125 times the number of Series J Preferred Units issued to the General Partner.

(d)         The General Partner is hereby amending Exhibit 1 to the Agreement by substituting for the existing addendum to Exhibit 1 the Addendum to

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Exhibit 1 in the form attached hereto to reflect the issuance of the Series J Preferred Units to the General Partner.

2.             Except as explicitly modified by this Amendment, all of the provisions of the Agreement are hereby ratified and confirmed, and shall remain in full force and effect.

3.             This Amendment shall take effect upon the contribution of the net proceeds of the Offering to the Partnership by the General Partner, and in the event such contribution is not made, this Amendment shall be of no force or effect.

(SIGNATURE PAGE FOLLOWS)

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In witness whereof, the General Partner has executed this Amendment as of the day and year first above written.

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Exhibit 1 Addendum

*                    Priority Return Percentage is expressed as a percentage of the Liquidation Preference per Distribution Period.  See the Agreement for the definitions of “Priority Return Percentage,” “Liquidation Preference” and “Distribution Period.”

**             Liquidation Preference Per Series I Preferred Unit shall equal $25.00 plus all accrued and unpaid distributions thereon.  In determining the Liquidation Preference, unpaid distributions shall accrue and be compounded on a quarterly basis.

***      Priority Return Percentage for the Series I Preferred Units shall be governed by Section 4 of the Seventeenth Amendment.  The Distribution Period for the Series I Preferred Units shall be each calendar quarter ending on March 31, June 30, September 30 and December 31, of each year.

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