<PAGE>

                                                                    EXHIBIT 31.1

           RULE 13A - 14 (a) CERTIFICATION OF CHIEF EXECUTIVE OFFICER

I, Thian Hoo Tan, certify that:

1. I have reviewed this annual report on Form 10-K of Komag, Incorporated;

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 and 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 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 registrant's board of
directors:

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

DATE March 16, 2005                             BY: /s/  Thian Hoo Tan
                                                    ------------------
                                                Thian Hoo Tan
                                                Chief Executive Officer