This paper examines the relationship between CEO overconfidence and product market competition. In product market competition, we focus on cost leadership strategy and market differentiation strategy. Using a sample of listed on TSE and OTC in Taiwan from 2001 to 2009 and the Panel data regression model, we find that an overconfident CEO will push businesses to engage in market differentiation strategy than cost leadership strategy. Our results suggest that the matching between manager's personal peculiarity and business strategy is important in successful business.