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                                                                    EXHIBIT 12.1

                                 CERTIFICATIONS

I, Tan Lay Koon, certify that:

1.       I have reviewed this annual report on Form 20-F of ST Assembly Test
         Services Ltd;

2.       Based on my knowledge, this annual 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 company as of, and for, the periods presented in this report;

4.       The company's other certifying officer(s) and I are responsible for
         establishing and maintaining disclosure controls and procedures (as
         defined in Exchange Act Rules 13a-15(e) and 15(e)) for the company 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 company, 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.       evaluated the effectiveness of the company'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

         c.       disclosed in this report any change in the company's internal
                  control over financial reporting that occurred during the
                  period covered by the annual report that has materially
                  affected, or is reasonably likely to materially affect, the
                  company's internal control over financial reporting; and

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

         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
                  company'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
                  company's internal control over financial reporting.

Date: March 19, 2004

/s/ Tan Lay Koon
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Tan Lay Koon
President and Chief Executive Officer