Audit expertise plays a significant role in determining, driving and explaining audit performance through audit specialization as a mediator. Hence, this study aims investigating the effects of audit expertise on audit specialization, examine the influences on audit expertise on audit performance, and test the impacts of audit specialization on audit performance. It also verifies whether audit specialization is a mediator of the audit expertise-audit performance relationships. The objective of this study is to investigate the relationships among audit expertise, audit specialization and audit performance of certified public accountants (CPAs) in Thailand. In this study, 113 CPAs in Thailand are the sample of the study. The results of this study indicate that audit expertise is positively related to audit specialization; audit expertise is positively associated with audit performance; and audit specialization is positively interacted with audit performance. In sum, audit expertise has both direct and indirect effects on audit performance; and audit specialization is a partial mediator of the audit expertise-audit performance relationships. To verify the results of the study and to expand the reliability and creditability of the study, future research is needed to search for other independent variables, mediating variables and moderating variables and include them in the conceptual model; and to collect data from a larger population and/or a comparative population or from other auditing professions