This paper studied the post M&A changes of corporate governance and firm value. Companies are able to enhance their corporate value by improving their corporate governance. For this purpose M&A can be measures of enhancing corporate value. This study investigates how corporate governance and value change after M&A. The result of this study is as follows. First, the overall impact of M&A on the corporate governance was not so great but post M&A public disclosure and Audit score has risen. This result means the improvement of board management and audit structure after M&A. Second, Long term effect of performance was estimated by Market-Model, CAPM, and Fama-French three factor model. In consequence abnormal return does not change uniformly and is not significant. But only positive CAAR change is significant under 5% significance level. Third, this study also investigated changes of financial variable before and after M&A. Assets and total assets turnover are shown to increase for the increasing group of CG score while does only assets for the falling group of CG score. Finally, this study shows that corporate governance and value are positively associated with firm value. This means that the improvement of board of directors and audit organization ultimately leads to post M&A increase of firm vlaue.