There have been a lot of researches especially on M&A announcement effects. However,what is more interest topic today is that why stock prices react significantly to the M&A announcement and a few studies argue that the agency theory is one of the main reasons. Especially, as many researches argue that firms with a good corporate governance system tend to have less agency problems than others, recent papers focus on the effects of M&A announcement from the perspective of agency theories by using corporate governance data as proxy variables. Moreover, these studies consistently suggest that if firms have a good corporate governance system, M&A announcement effects in those firms are higher than counterparts significantly. However, different from prior research, studies in Korea do not show that the level of governance system tends to have positive relation with market reactions to M&A announcements. The most important reason why there are not consistent results with respect to the effects of agency problems to M&A is that there are not so good proxy variables for the degree of agency costs. In other words, even though a firm’s corporate governance system can be affected by many factors including ownership structures, prior researches in Korea usually use just ownership data such as manager’s ownership ratios or institutional ratios as proxy variables. Meanwhile, since the IMF economic crisis, Korean firms’ corporate governance systems have been considered as the main reason for the so-called Korean Discount. So, there are many changes in regulations such as the board of directors and audit committee and so on. Additionally, the important roles of the board of directors and audit committee are to increase the efficiency of firm’s investment activities including M&A. Therefore, after the IMF crisis, we can guess that in Korea the effects of corporate governance, especially board of directors and audit committee became more important. Unlike the Masulis et al.(2007) paper which focuses on anti-takeover provisions, this paper investigates the effects of corporate governance on M&A announcements based on the agency theories by using the KSE listing firms over the period between 2003 and 2008. In particular, this paper uses individual corporate governance scores provided by the KCGS to test the effects of governance system on the market reactions. As prior studies argue, the KCGS’s governance scores are likely to be good proxy variables for the level of a firm's agency problems because there are a few results that show the positive relations between corporate governance scores and firm value. Considering all reasons mentioned above, we can find that generally firms with good corporate governance systems tend to have significantly positive abnormal returns than others, which means that corporate governance systems can mitigate agency problems by controling manager’s inefficient behaviors. In addition, in subsample tests, we also find that the board of directors and audit committee in Korean firms are the most important factors to affect the market reactions to M&A announcements. Finally, this paper shows that the effects of two abovementioned systems can be work even in business groups, Chaebols, if Chaboles have good corporate governance systems, which means unlike the prior studies, the Chaebol variable is not a good proxy variable for the level of agency costs. In conclusion, these results are consistent with the prior research results based on the agency theories. Moreover, especially in Korea, the board of directors and audit committee are more important factors to affect the M&A announcement events, resulting in increasing firm value significantly.