his study examines the relationship between corporate governance and the announcement effect of operational loss. The occurrence of operational loss is associated with unsound corporate governance mechanism and influents firm’s stock price negatively. We study 540 operational events in US market during 1994 and 2009. The empirical results show that internal corporate governance mechanism affects market reaction on operational loss announcement. In general, there is a positive association between percentage of institutional holdings and market reaction. We also find that internal corporate governance factors have different influences among industries. Contrary to the importance of internal corporate governance, external governance mechanism is not a crucial determinant of operational loss announcement effect Furthermore, for those events caused by internal fraud, corporate governance seems irrelevant.