<PAGE>

                                                                   EXHIBIT 10.11

                            SIXTH AMENDMENT TO THE
                        MANTECH INTERNATIONAL 401(k) PLAN

         SIXTH 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.

         This Amendment of the Plan is adopted to reflect certain provisions of
the Economic Growth and Tax Relief Reconciliation Act of 2001 ("EGTRRA"). This
Amendment is intended as good faith compliance with the requirements of EGTRRA
and is to be construed in accordance with EGTRRA and the guidance issued
thereunder. Except as otherwise provided, this Amendment shall be effective as
of the first day of the first Plan Year beginning after December 31, 2001. This
amendment shall supersede the provisions of the Plan to the extent those
provisions are inconsistent with the provisions of this Amendment.

         NOW THEREFORE, the Plan is amended as follows:

I.       Section 1.7 of the Plan is amended by adding a new subsection (e) as
         follows:

         "(e) Increase in Compensation Limit. The annual compensation of each
         Participant taken into account in determining allocations for any Plan
         Year beginning after December 31, 2001, shall not exceed $200,000, as
         adjusted for cost-of-living increases in accordance with Code section
         401(a)(17)(B). Annual compensation means compensation during the Plan
         Year or such other consecutive 12-month period over which compensation
         is otherwise determined under the Plan (the determination period). The
         cost-of-living adjustment in effect for a calendar year applies to
         annual compensation for the determination period that begins with or
         within such calendar year."

<PAGE>

II.      Section 1.39 of the Plan is amended in its entirety to read as follows:

         "1.39    Top Heavy:

                  (a)  A plan is Top Heavy if it is one of one or more plans
         maintained by the Employer that are qualified under Code section 401(a)
         and under which the sum of the present values of accrued benefits of
         Key Employees under defined benefit plans and the account balances of
         Key Employees under defined contribution plans exceeds 60% of the sum
         of the present values of accrued benefits and account balances of all
         employees, former employees (except former employees who performed no
         services for the Employer for the five-year period ending on the
         determination date), and beneficiaries in the plans. The "determination
         date" is the date on which it is determined whether this Plan is Top
         Heavy. Such determination shall be made as of the last day of the
         immediately preceding Plan Year and shall be made in accordance with
         Code section 416(g). If the Employer and Related Companies maintain
         more than one plan qualified under Code section 401(a), then (a) each
         such plan in which a Key Employee is a participant, and (b) each such
         plan that must be taken into account in order for a plan described in
         the preceding clause to meet the requirements of Code section 401(a)(4)
         or 410 shall be aggregated with this Plan to determine whether the
         plans, as a group, are Top Heavy. The Employer and Related Companies
         may aggregate any other qualified plan with this Plan to the extent
         that such aggregation is permitted by Code section 416(g). For purposes
         of the preceding sentence, a plan includes a terminated plan which was
         maintained by the Employer within the last five years ending on the
         determination date and which would otherwise be required to be
         aggregated with this Plan.

                  (b)  Modification of Top Heavy Rules

                       (i)   Effective date. This subsection (b) shall apply for
         purposes of determining whether the Plan is a top heavy plan under Code
         section 416(g) for Plan Years beginning after December 31, 2001, and
         whether the Plan satisfies the minimum benefits requirements of Code
         section 416(c) for such years. This subsection (b) amends Sections
         1.39(a) and 1.21 of the Plan.

                       (ii)  Determination of top heavy status.

                             (A) Key employee. Key employee means any employee
         or former employee (including any deceased employee) who at any time
         during the Plan Year that includes the determination date was an
         officer of the employer having annual compensation greater than
         $130,000 (as adjusted under Code section 416(i)(1) for Plan Years
         beginning after December 31, 2002), a 5-percent owner of the employer,
         or a 1-percent owner of the employer having annual compensation of more
         than $150,000. For this purpose, annual compensation means compensation
         within the meaning of Code section 415(c)(3). The determination of who
         is a key employee will be made in accordance with Code section
         416(i)(1) and the applicable regulations and other guidance of general
         applicability issued thereunder.

                                       2

<PAGE>

                             (B)  Determination of present values and amounts.
         This subsection (B) shall apply for purposes of determining the present
         values of accrued benefits and the amounts of account balances of
         employees as of the determination date.

                                 (1) Distributions during year ending on the
         determination date. The present values of accrued benefits and the
         amounts of account balances of an employee as of the determination date
         shall be increased by the distributions made with respect to the
         employee under the Plan and any plan aggregated with the Plan under
         Code section 416(g)(2) during the 1-year period ending on the
         determination date. The preceding sentence shall also apply to
         distributions under a terminated plan which, had it not been
         terminated, would have been aggregated with the Plan under Code section
         416(g)(2)(A)(i). In the case of a distribution made for a reason other
         than separation from service, death, or disability, this provision
         shall be applied by substituting "5-year period" for "1-year period."

                                 (2) Employees not performing services during
         year ending on the determination date. The accrued benefits and
         accounts of any individual who has not performed services for the
         employer during the 1-year period ending on the determination date
         shall not be taken into account.

                       (iii) Minimum benefits.

                          (A) Matching contributions. Company Basic Matching
         Contributions and Company Supplemental Matching Contributions shall be
         taken into account for purposes of satisfying the minimum contribution
         requirements of Code section 416(c)(2) and the Plan. These matching
         contributions that are used to satisfy the minimum contribution
         requirements shall be treated as matching contributions for purposes of
         the actual contribution percentage test and other requirements of Code
         section 401(m).

                          (B) Contributions under other plans. The minimum
         contribution required under Code section 416(c)(2) shall be made under
         this Plan to the extent the minimum contribution is not otherwise
         provided under any other defined contribution plan maintained by the
         Employer or a Related Company, or by the Company contributions for the
         Plan Year under this Plan."

III.     Effective January 1, 2002, Section 3.2(b) of the Plan and the paragraph
         immediately thereafter are amended in their entirety to read as
         follows:

         "(b) No Participant shall be permitted to have Deferral Contributions
         under this Plan, or any other qualified plan maintained by the Employer
         during any taxable year, in excess of the dollar limitation contained
         in Code section 402(g) in effect for such taxable year, except to the
         extent permitted under Section 3.4A of this Plan and Code section
         414(v), if applicable.

                                       3

<PAGE>

         Deferral Contributions must be made in whole percentages of
         Compensation, unless the Plan Administrator permits otherwise. If a
         Participant participates in another plan that is subject to the limit
         of subsection (b), above, the Participant may allocate any
         contributions in excess of such limit among the plans in which he
         participates."

IV.      Effective January 1, 2002, the Plan is amended by adding a new Section
         3.4A as follows:

         "3.4A    Catch Up Contributions:

                  (a) All employees who are eligible to make Deferral
         Contributions under this Plan and who have attained age 50 before the
         close of the Plan Year shall be eligible to make catch-up contributions
         in accordance with, and subject to the limitations of, Code section
         414(v). Such catch-up contributions shall not be taken into account for
         purposes of the provisions of the Plan implementing the required
         limitations of Code sections 402(g) and 415. The Plan shall not be
         treated as failing to satisfy the provisions of the Plan implementing
         the requirements of Code sections 401(k)(3), 401(k)(11), 401(k)(12),
         410(b), or 416, as applicable, by reason of the making of such catch-up
         contributions.

                  (b) This Section 3.4A shall apply to contributions after
         December 31, 2001."

V.       Section 4.5(a) of the Plan is amended in its entirety to read as
         follows:

         "(a)     (i) Notwithstanding any other provisions of the Plan,
         contributions and other additions with respect to a Participant exceed
         the limitation of Code section 415(c) if, when expressed as an Annual
         Addition (within the meaning of Code section 415(c)(2)) to the
         Participant's account, such Annual Addition is greater than the lesser
         of: (A) $30,000 or (B) 25% of the Participant's Section 415
         Compensation.

                  (ii)Limitations on Contributions.

                      (A) Effective date. This subsection (ii) shall be
         effective for Limitation Years beginning after December 31, 2001.

                      (B) Maximum annual addition. Except to the extent
         permitted under Section 3.4A of the Plan and Code section 414(v), if
         applicable, the annual addition that may be contributed or allocated to
         a Participant's Account under the Plan for any Limitation Year shall
         not exceed the lesser of (1) $40,000, as adjusted for increases in
         cost-of-living under Code section 415(d), or (2) 100 percent of the
         Participant's compensation, within the meaning of Code section
         415(c)(3), for the Limitation Year. The compensation limit referred to
         in (2) shall not apply to any contribution for medical benefits after
         separation from service (within the meaning of Code section 401(h) or
         419A(f)(2)) which is otherwise treated as an annual addition."

                                       4

<PAGE>

VI       Section 4.8(e) of the Plan is amended by adding the following sentence
to the end thereof:

         "The multiple use test described in Treasury Regulation section
         1.401(m)-2 and this Section 4.8(e) of the Plan shall not apply for Plan
         Years beginning after December 31, 2001."

VII.     Effective January 1, 2002, Section 6.7 of the Plan is amended by adding
a new subsection (g) as follows:

         "(g) Distribution upon Severance from Employment.

              (i)  Effective date. This subsection (g) shall apply for
         distributions and severances from employment occurring after December
         31, 2001.

              (ii) New distributable event. A Participant's elective
         deferrals, qualified nonelective contributions, qualified matching
         contributions, and earnings attributable to these contributions shall
         be distributed on account of the Participant's severance from
         employment. However, such a distribution shall be subject to the other
         provisions of the Plan regarding distributions, other than provisions
         that require a separation from service before such amounts may be
         distributed."

VIII.    Effective January 1, 2002, Section 6.10(c) of the Plan is amended by
adding the following paragraph to the end thereof:

         "A Participant who receives a distribution of elective deferrals after
         December 31, 2001, on account of hardship shall be prohibited from
         making elective deferrals and employee contributions under this and all
         other plans of the Employer for six months after receipt of the
         distribution. A Participant who receives a distribution of elective
         deferrals in calendar year 2001 on account of hardship shall be
         prohibited from making elective deferrals and employee contributions
         under this and all other plans of the Employer for six months after
         receipt of the distribution or until January 1, 2002, if later."

IX.      Effective January 1, 2002, Section 6.14 of the Plan is amended by
adding a new subsection (c) as follows:

         "(c) Direct Rollovers of Plan Distributions.

              (i)  Effective date.  This subsection (c) shall apply to
         distributions made after December 31, 2001.

                                       5

<PAGE>

                   (ii)  Modification of definition of eligible retirement plan.
         For purposes of the direct rollover provisions in this Section 6.14 of
         the Plan, an eligible retirement plan shall also mean an annuity
         contract described in Code section 403(b) and an eligible plan under
         Code section 457(b) which is maintained by a state, political
         subdivision of a state, or any agency or instrumentality of a state or
         political subdivision of a state and which agrees to separately account
         for amounts transferred into such plan from this Plan. The definition
         of eligible retirement plan shall also apply in the case of a
         distribution to a surviving spouse, or to a spouse or former spouse who
         is the alternate payee under a qualified domestic relation order, as
         defined in Code section 414(p).

                   (iii) Modification of definition of eligible rollover
         distribution to exclude hardship distributions. For purposes of the
         direct rollover provision in this Section 6.14 of the Plan, any amount
         that is distributed on account of hardship shall not be an eligible
         rollover distribution and the distributee may not elect to have any
         portion of such a distribution paid directly to an eligible retirement
         plan.

                   (iv)  Modification of definition of eligible rollover
         distribution to include after-tax employee contributions. For purposes
         of the direct rollover provisions in this Section 6.14 of the Plan, a
         portion of a distribution shall not fail to be an eligible rollover
         distribution merely because the portion consists of after-tax employee
         contributions which are not includible in gross income. However, such
         portion may be transferred only to an individual retirement account or
         annuity described in Code section 408(a) or 408(b), or to a qualified
         defined contribution plan described in Code section 401(a) or 403(a)
         that agrees to separately account for amounts so transferred, including
         separately accounting for the portion of such distribution which is
         includible in gross income and the portion of such distribution which
         is not so includible."

X.       Effective January 1, 2002, Section 13.1 of the Plan is amended by
adding a new subsection (d) as follows:

         "(d) Rollovers from Other Plans. The Plan will accept a direct rollover
         of an eligible rollover distribution from a qualified plan described in
         Code sections 401(a) or 403(a), including after-tax employee
         contributions; an annuity contract described in Code section 403(b),
         excluding after-tax employee contributions; and an eligible plan under
         Code section 457(b) which is maintained by a state, political
         subdivision of a state, or any agency or instrumentality of a state or
         political subdivision of a state. The Plan will accept a participant
         contribution of an eligible rollover distribution from a qualified plan
         described in Code sections 401(a) or 403(a); an annuity contract
         described in Code section 403(b); and an eligible plan under Code
         section 457(b) which is maintained by a state, political subdivision of
         a state, or any agency or instrumentality of a state or political
         subdivision of a state. The Plan will accept a participant rollover
         contribution of the portion of a distribution from an individual
         retirement account or annuity described in Code section 408(a) or
         408(b) that is eligible to be rolled over and would otherwise be
         includible in gross income. This subsection (d), Rollovers from Other
         Plans, shall be effective January 1, 2002."

                                       6

<PAGE>

XI.      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 ___________, 2002.

                                       Mantech International Corporation

                                       By: _____________________________________

                                       7