**Exhibit 31.1****RULE13a — 14 (a) CERTIFICATION OF CHIEF EXECUTIVE OFFICER**I, Thian Hoo Tan, certify that:1. I have reviewed this quarterly report on Form 10-Q of Komag, Incorporated;2. Based on my knowledge, this report does not contain any untrue statement of a material fact oromit to state a material fact necessary to make the statements made, in light of the circumstancesunder which such statements were made, not misleading with respect to the period covered by thisreport;3. Based on my knowledge, the financial statements and other financial information included in thisreport fairly present, in all material respects, the financial condition and results of operationsand 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 maintainingdisclosure 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) and15d — 15 (f)) for the registrant and have:a) Designed such disclosure controls and procedures, or caused such disclosure controls andprocedures to be designed under our supervision, to ensure that material information relating tothe registrant, including its consolidated subsidiaries, is made known to us by others within thoseentities, particularly during the period in which this report is being prepared;b) Designed such internal control over financial reporting, or caused such internal control overfinancial reporting to be designed under our supervision, to provide reasonable assurance regardingthe reliability of financial reporting and the preparation of financial statements for externalpurposes in accordance with generally accepted accounting principles;c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presentedin 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; andd) Disclosed in this report any change in the registrant’s internal control over financialreporting that occurred during the registrant’s most recent fiscal quarter (the registrant’s fourthfiscal quarter in the case of an annual report) that has materially affected, or is reasonablylikely to materially affect, the registrant’s internal control over financial reporting; and5. The registrant’s other certifying officer and I have disclosed, based on our most recentevaluation of internal control over financial reporting, to the registrant’s auditors and the auditcommittee of registrant’s board of directors:a) All significant deficiencies and material weaknesses in the design or operation of internalcontrol over financial reporting which are reasonably likely to adversely affect the registrant’sability to record, process, summarize, and report financial information; andb) Any fraud, whether or not material, that involves management or other employees who have asignificant role in the registrant’s internal control over financial reporting.      DATE: August 5, 2005 BY: /s/ Thian Hoo Tan            Thian Hoo Tan  Chief Executive Officer