This study examines the relationship between CEO overconfidence and corporate investment after deregulation. Industry deregulation allows deregulated firms more operation freedom, gives managers more discretion over operation strategies, and creates a more competitive environment. Therefore, managerial function or decision quality become more and more important after deregulation. Our empirical findings suggest that overconfident CEOs of deregulated firms are more willing to conduct investment, in particular advertising expense, in the post-deregulation period. Furthermore, the deregulated firms with overconfidence CEOs perform better than other matched firms after complete deregulation.