In an era of knowledge economy, it is worthwhile to explore the topic of principals' knowledge leadership. Little research explores the potential mediating mechanism on the association between knowledge leadership and organizational effectiveness. This study identifies the teachers' professional learning community as a potential mediating variable. This study aimed to investigate the relationships between elementary school principals' knowledge leadership, teachers' professional learning community, and school effectiveness. This study adopted questionnaire survey method. The research samples were the teachers at public municipal elementary schools in Kaohsiung City. Multiple regression analysis was used to verify hypotheses. The findings indicated that principals' knowledge leadership and teachers' professional learning community had positive and significant effects on organizational effectiveness. Principals' knowledge leadership had positive and significant effect on teachers' professional learning community. Besides, teachers' professional learning community partly mediated the relationship between principals' knowledge leadership and organizational effectiveness. Based on these findings, this study proposed conclusions and practical suggestions for schools and teachers.