This paper examines the effect of board gender diversity on firm performance in China's listed firms from 1999 to 2011. We document a positive and significant relation between board gender diversity and firm performance. Female executive directors have a stronger positive effect on firm performance than female independent directors, indicating that the executive effect outweighs the monitoring effect. Moreover, boards with three or more female directors have a stronger impact on firm performance than boards with two or fewer female directors, consistent with the critical mass theory. Finally, we find that the impact of female directors on firm performance is significant in legal person-controlled firms but insignificant in state-controlled firms. This paper sheds new light on China's boardroom dynamics. As governments increasingly contemplate board gender diversity policies, our study offers useful empirical guidance to Chinese regulators on the issue.