EXHIBIT 5.1

                                                       VINSON & ELKINS L.L.P.
                                                       3700 TRAMMELL CROW CENTER
                                                       2001 ROSS AVENUE
                                                       DALLAS, TEXAS  75201-2975
                                                       TELEPHONE (214) 220-7700
                                                       FAX (214) 220-7716
                                                       www.velaw.com

August 9, 2006

Pioneer Natural Resources Company
5205 North O'Connor Boulevard
Suite 200
Irving, Texas  75039

Ladies and Gentlemen:

     We have acted as counsel for Pioneer Natural Resources  Company, a Delaware
corporation (the "Company"), in connection with the Company's registration under
the  Securities  Act of 1933,  as amended  (the "Act") of 200,000  shares of the
Company's  common  stock,  par value  $0.01 per share and  associated  rights to
purchase shares of Series A Junior Participating Preferred Stock (the "Shares"),
pursuant to the Company's  registration statement on Form S-8 (the "Registration
Statement") filed with the Securities and Exchange Commission (the "Commission")
on August 10, 2006.

     In  reaching  the  opinions  set forth  herein,  we have  examined  and are
familiar  with  originals or copies,  certified or otherwise  identified  to our
satisfaction,  of such  documents and records of the Company and such  statutes,
regulations  and other  instruments  as we deemed  necessary  or  advisable  for
purposes of this opinion,  including (i) the  Registration  Statement,  (ii) the
amended and restated  certificate of incorporation of the Company, as filed with
the Secretary of State of the State of Delaware,  (iii) the amended and restated
bylaws of the  Company,  and (iv)  certain  resolutions  adopted by the board of
directors of the Company.

     We have assumed  that (i) all  information  contained  in all  documents we
reviewed is true, correct and complete,  (ii) all signatures on all documents we
reviewed are genuine,  (iii) all documents submitted to us as originals are true
and complete, (iv) all documents submitted to us as copies are true and complete
copies of the originals  thereof,  and (v) all persons  executing and delivering
the documents we examined were competent to execute and deliver such  documents.
In addition,  we have  assumed that (i) the Shares will be issued in  accordance
with the terms of the Pioneer  Natural  Resources  Company  Long-Term  Incentive
Plan, as amended (the "Plan"),  (ii) the full consideration for each Share shall
be paid to the  Company and in no event will be less than the par value for each
Share, and (iii)  certificates  evidencing the Shares will be properly  executed
and delivered by the Company in accordance with the Delaware General Corporation
Law.

     Based on the foregoing,  and having due regard for the legal considerations
we deem  relevant,  we are of the opinion  that the  Shares,  when issued by the
Company in accordance with the terms of the Plan, will be legally issued,  fully
paid and non-assessable.

     This  opinion is limited in all respects to the laws of the States of Texas
and Delaware and the federal laws of the United States of America, and we do not
express any opinion as to the laws of any other jurisdiction.

     This  opinion  letter  may be  filed  as an  exhibit  to  the  Registration
Statement.  In giving this consent,  we do not thereby admit that we come within
the category of persons whose consent is required  under Section 7 of the Act or
the rules and regulations of the Commission promulgated thereunder.

                                Very truly yours,

                                /s/ Vinson & Elkins L.L.P.