From the financial behavior perspective, firms use mergers and acquisitions in order to raise company value. This thesis aims to investigate CEO overconfidence in China companies and test whether or not CEOs overconfidence will have an impact on firms’ value after mergers and acquisitions. Because of the balance of power conflict might arise at a firm after mergers and acquisitions, thus, even corporate governance is a way to monitor CEO power, however, the internal conflict and authorities conflict may arise after two companies have merged into one corporate and have negative impact on firm value after mergers and acquisitions. The empirical results of this thesis have shown that firm value is positively related to CEOs overconfidence. In addition, the overconfidence of young CEOs show significant more aggressive in mergers and acquisitions, thus firm value has enhanced because of young CEOs eager to success in the early career stage. Also, this thesis evidence shows that if firms prefer to focus more in supervisory and risk aversion control, female CEOs are the ideal candidate for the establishment the corporate policy and risk management. Also, CEO who possesses a bachelor degree and financial background shows a significant and positive influence to slow down firm value decreased in the empirical result.