This study explored how a group of higher education teachers in China used WeChat to build an online Community of Practice (OCoP), which supported their professional learning. A qualitative study was designed. Data were collected from multiple sources including online interaction excerpts, teachers' reflections, classroom observation, and interviews. The results showed that the participants were utilizing the teacher group created through WeChat to build an OCoP for professional learning in terms of three dimensions: mutual engagement, joint enterprise, and shared repertoire. Positive perceptions were identified in these participants on joining the teacher group. Moreover, their teaching practices were transformed positively. A framework for using mobile social media to transform OCoPs into knowledge-building communities for teacher professional learning was proposed based on the findings. Guidelines for nurturing and maintaining OCoPs as well as for adopting the framework are developed. Suggestions for future research are provided.