The present study examined how a group of Hong Kong in-service teachers improved their knowledge of how to promote self-regulated learning (SRL) in students when they participated in a teacher professional development program, which was designed according to the principles of Communities of Practice (CoP). A case study method design was employed and multiple sources of data were collected to investigate the teacher participants' improvement in the knowledge of SRL-based instruction. Findings showed that, over the period of six months, most teachers changed their knowledge of SRL-based instruction from providing students with full autonomy in self-studying a topic to strengthening teachers' role in creating a high-SRL classroom environment. Apart from the CoP experience, evidence of students' involvement in learning was also important to change the teachers' understanding of SRL-based instruction. Discussion and suggestions are provided at the end. Copyright © 2019 Elsevier Ltd. All rights reserved.