This paper aims to explore the mechanism of Learning Study in enhancing teacher learning by adopting community of practices as the analytical framework. Under the curriculum reform in Hong Kong, schools are facing challenges in acquiring pedagogical content knowledge for implementing new curriculum. How to enhance teacher learning for building school capacity is critical for sustainable development. Learning Study is a school improvement project which aims to improve the quality of student learning via enhancing teacher professional development by creating a community of practice among teachers, researchers and educators involved in the design, implementation, evaluation and dissemination of a research lesson, with ultimate goal of developing Learning Community in the school. This study will explore how the community of practice set up by the Learning Study Project can effectively help teachers build their pedagogical content knowledge. Wenger's theory of Communities of Practices, which postulate that participants have a mutual engagement for a negotiated joint enterprise and they develop a shared repertoire, is adopted as the theoretical framework for exploring teacher learning and professional growth. The study adopts ethnographic approach to investigate social phenomena of teacher interactions in a case selected from Learning Study Project. Results show that the collaborative features of the Learning Study provide opportunities for teachers to create shared knowledge, values and beliefs and to establish overlapping histories among them. Learning Study enhances teacher learning by creating a community of practice for knowledge sharing and reflective practices on teaching and learning.