This paper investigates the effect of product market competition, along with the interaction effect of market competition and the type of mergers and acquisitions (focused or diversified M&As) on an acquirer's post-merger long-term performance. Literature suggests that market competition has the function of disciplining managers and forcing them to operate efficiently. Our main hypotheses are that market competition has a positive effect on post-merger performance only for focused mergers due to the diversification-discount phenomenon, and that diversified mergers may erode the benefits of competition in post-merger performance. Using merger events for Taiwan's listed firms as a sample, the evidence supports our hypotheses, even when considering the endogeneity of market competition and considering only non-electronics events. The discipline derived from market competition tends to be weak across the whole sample. The robustness check confirms the limited positive effect of competition caused by finding a negative relation between acquisition premium and competition only for the focused M&As.