According to the literature on entrepreneurial orientation (EO),proactive firms are more likely to achieve first-mover advantage and higher performance. The neoinstitutionalism, however, suggests that enterprises with more legitimacy will acquire more growth opportunities. Usually, the first mover might face more legitimacy obstacles. To date, there has been little research on how proactive firms cope with legitimacy constraints and achieve firm growth.Integrating the legitimacy perspective and the EO literature, this research examines the roles of ISO certification as a strategy for seeking legitimacy of small and medium-sized enterprises (SMEs) in emerging economies, and the relationship between proactiveness and firm performance. In particular, it hypothesizes that ISO certification has a mediating effect on the relationship between proactiveness and firm growth. We test the hypothesis using a sample of 632 firms collected from a nationwide survey on SMEs conducted by the Chinese SME Association. The results reveal that ISO certification partially mediates the relationship between proactiveness and firm growth, suggesting that proactive firms tend to use legitimation via ISO certification to enhance firm growth. Our paper contributes to the literature by shedding light on the important relationship between seeking legitimacy, entrepreneurial orientation and firm growth in SMEs in an emerging economy.