<PAGE>

                                                                  EXHIBIT 10.8

                             THIRD AMENDMENT TO THE
                        MANTECH INTERNATIONAL 401(k) PLAN

         AMENDMENT to the ManTech International 401(k) Plan (the "Plan") by
ManTech International Corporation (the "Company").

         The Company maintains the Plan, effective as of January 1, 1993, and
amended and restated effective as of October 1, 1998. The Company has the power
to amend the Plan and now wishes to do so.

         NOW THEREFORE, the Plan is amended as follows:

I.       Section 2.1(b) of the Plan is amended in its entirety to read as
         follows:

         (b) Effective January 1, 2001, each Eligible Employee who is not
         already a Participant pursuant to subsection (a) will become a
         Participant for purposes of Deferral Contributions, Participant
         Voluntary Contributions and Participant Basic After-Tax Contributions
         under Sections 3.2 and 3.3 on the first Entry Date following the date
         on which he completes one Hour of Service. Each Eligible Employee who
         is not already a Participant pursuant to subsection (a) will become a
         Participant for purposes of Company Basic Matching Contributions and
         Fail Safe Contributions under Sections 3.4 and 3.5 as of the first
         Entry Date following the date on which he has completed three months of
         employment with the Employer.

II.      Section 3.3(c) of the Plan is amended in its entirety to read as
         follows:

         (c) Effective January 1, 2001, in lieu of Deferral Contributions and
         Voluntary Contributions, an Overseas Participant shall be eligible to
         make Participant Basic After-Tax Contributions to the Trust Fund. An
         Overseas Participant shall not be entitled to claim a tax deduction for
         his Participant Basic After-Tax Contributions. The amount of
         Participant Basic After-Tax Contributions made by any Overseas
         Participant to this Plan shall be a designated percentage, from 1% to
         20% of the Participant's Compensation. An Overseas Participant's total
         Deferral Contributions and Participant Basic After-Tax Contributions
         shall not exceed 20% of the total Compensation received by such
         Participant during the Plan Year. Participant Basic After-Tax
         Contributions shall be administered on the same basis as Participant
         Deferral Contributions and shall be subject to the provisions of
         Section 3.3(b).

                                       1

<PAGE>

III.     The first sentence of Section 3.4(a) of the Plan is amended in its
         entirety to read as follows:

         (a) For each Plan Year, the Company will make a Company Basic Matching
         Contribution on behalf of each of its Participants who have elected to
         have Deferral Contributions made on their behalf during the Plan Year,
         except that, anything else in this subsection notwithstanding, the
         Company will not make a Company Basic Matching Contribution with
         respect to Deferral Contributions made before a Participant became a
         Participant for purposes of Company Basic Matching Contributions under
         Section 2.1(b) of the Plan.

IV.      Section 3.4(b) is amended in its entirety to read as follows:

         (b) In lieu of the Contribution made under subsection (a) above,
         Overseas Participants will receive a Company Basic Matching
         Contribution equal to 50% of the first 4% of the Compensation that the
         Participant defers as either a Deferral Contribution or a Participant
         Basic After-Tax Contribution during the Plan Year, except that,
         anything else in this subsection notwithstanding, the Company will not
         make a Company Basic Matching Contribution with respect to Deferral
         Contributions or After-Tax Contributions made before a Participant
         became a Participant for purposes of Company Basic Matching
         Contributions under Section 2.1(b) of the Plan.

V.       In all respects not amended, the Plan is hereby ratified and confirmed.

                                    * * * * *

         To record adoption of the Amendment as set forth above, the Company has
caused this document to be signed on this ____ day of ____________________.

                                               MANTECH INTERNATIONAL CORPORATION

                                               BY: _____________________________

                                       2