This study aims to investigate the effect size between teachers' leadership and school effectiveness. Additionally, it is aimed to suggest implications for policy. In this study, a meta-analysis was conducted to examinate the effect size. A total of eighty including theses, dissertations, and journal articles on the relationship between teachers’ leadership and school effectiveness, published from 1980 to 2011, were analyzed. The results of this study are as follows; First, the relationship between teachers’ leadership and school effectiveness shows a statistically significant effect size. Second, teachers’ leadership has a greater impact on the present, a primary school level, and a classroom­wise rather than the past, a secondary school level, and a student­wise, respectively. Third, transformational leadership is most meaningful in affecting school effectiveness. The results indicate the following policy implications. First, teachers’ leadership should be put on the first priority in order to improve school effectiveness. That is why it has positive impact on improving students’ intellectual and emotional development as well as on creating the ideal classroom setting. Second, as it is necessary to provide discretion for individual teachers, policy makers are required to create the empowerment system for teachers. Last, the programs for developing teachers’ leadership should be designed with taking account of various types of leadership and differences of school levels.