Based on the World Bank Investment Climate Survey, this paper investigates the openness effects on the efficiency of firms in China’s manufacturing industry using a two-step data envelopment analysis (DEA) approach. In the first step, the aggregate efficiency of open firms and non-open firms is compared in each sub-industry using a group-wise heterogeneous bootstrap procedure. The results show, at a 90% confidence level, that open firms are more efficient than non-open firms in four out of five sub-industries. Furthermore, in the second step, we employ the two-stage bootstrap DEA approach to more specifically evaluate the effects of openness on the efficiency of firms. The regression results show that three openness indicators (foreign capital, import and export) have strong positive effects on firms’ efficiency in China’s manufacturing industry. In addition, the results also suggest that a larger state share, larger firm size, and more capital stock are negatively related to the efficiencies of firms, while a firms’ learning and absorptive capacity is positively related to its efficiency.