Many empirical researches examined the size of board of directors and firm performance, but the conclusions were not coincident and the most researchers would  consider  joint endogeneity of variables. We contribute to this literature by exploring whether the size of the board of directors constitutes an independent governance mechanism was explored and was related to firm performance. Using a comprehensive set of listed Chinese companies, it is suggested that the size of the board of directors is an independent control mechanism. There is no significant relationship between board size and firm valuation. This suggests that Chinese firms，on average，choose their number of board members just optimally. Cross-sectional variations in board size reflect differences in firms' underlying environment，but not mistaken choices.