Professional Learning Communities (PLCs) are an important strategy for innovation in schools, and they are receiving considerable attention from scholars and educators alike. The present study aimed to examine the effect of PLCs on schools' effectiveness and to investigate the social, organizational, and structural factors that can promote these learning communities. The survey for this study was completed by 375 teachers from 40 elementary schools in the Seoul Metropolitan Area of South Korea, and their responses were analyzed to test the hypothesized model. The results of the structural equation modeling indicated that PLCs were strongly and directly related to elementary schools' effectiveness and that principals' leadership and supportive relationships among teachers were the important factors that influenced PLCs. Based on the results of this study, several implications are discussed.