EXHIBIT 31.1

CERTIFICATIONS

I, Marvin F. Romanow, certify that:

1.   I have reviewed this quarterly report on Form 10-Q of Nexen Inc.

2.   Based on my knowledge, this report does not contain any untrue statement of
     a material  fact or omit to state a  material  fact  necessary  to make the
     statements made, in light of the circumstances  under which such statements
     were made,  not  misleading  with  respect  to the  period  covered by this
     report;

3.   Based on my  knowledge,  the  financial  statements,  and  other  financial
     information  included  in  this  report,  fairly  present  in all  material
     respects the financial  condition,  results of operations and cash flows of
     the registrant as of, and for, the periods presented in this report;

4.   The  registrant's  other  certifying  officer  and  I are  responsible  for
     establishing and maintaining disclosure controls and procedures (as defined
     in exchange Act Rules  13a-15(e) and 15d-15(e))  and internal  control over
     financial  reporting  (as  defined  in  Exchange  Act Rules  13a-15(f)  and
     15d-15(f)) for the registrant and we have:

     (a)  designed  such  disclosure  controls  and  procedures,  or caused such
          disclosure   controls  and   procedures  to  be  designed   under  our
          supervision,  to ensure  that  material  information  relating  to the
          registrant,  including its consolidated subsidiaries, is made known to
          us by others within those entities,  particularly during the period in
          which this report is being prepared;

     (b)  designed such internal  control over  financial  reporting,  or caused
          such internal  control over  financial  reporting to be designed under
          our  supervision,   to  provide  reasonable  assurance  regarding  the
          reliability  of financial  reporting and the  preparation of financial
          statements for external purposes in accordance with generally accepted
          accounting principles;

     (c)  evaluated the  effectiveness of the registrant's  disclosure  controls
          and procedures and presented in this report our conclusions  about the
          effectiveness of the disclosure controls and procedures, as of the end
          of the period covered by this report based on such evaluation; and

     (d)  disclosed  in this  report  any  change in the  registrant's  internal
          control over financial reporting that occurred during the registrant's
          most recent fiscal quarter (the registrant's  fourth fiscal quarter in
          the case of an annual  report)  that has  materially  affected,  or is
          reasonably  likely to materially  affect,  the  registrant's  internal
          control over financial reporting; and

5.   The registrant's  other certifying  officer and I have disclosed,  based on
     our most recent evaluation of internal control over financial reporting, to
     the registrant's auditors and the audit committee of the registrant's board
     of directors (or persons performing the equivalent function):

     (a)  any  fraud,  whether  or all  significant  deficiencies  and  material
          weaknesses  in the  design  or  operation  of  internal  control  over
          financial  reporting which are reasonably  likely to adversely  affect
          the  registrant's  ability to record,  process,  summarize  and report
          financial information; and

     (b)  not material,  that involves  management or other employees who have a
          significant role in the  registrant's  internal control over financial
          reporting.

     Date: April 30, 2010

                                           /S/ MARVIN F. ROMANOW
                                          -------------------------------------
                                                  Marvin F. Romanow
                                          President and Chief Executive Officer