Teacher professional development has been considered as the most effective strategy to promote teacher change. However, this mostly presupposes that teacher professional development programmes can change teachers' attitudes and beliefs, subsequently change classroom behaviours and practices, and ultimately improve student learning outcomes. This article reports a case of a learning study of secondary school students learning mathematics in Hong Kong. It appeared that the learning study helped to improve student learning outcomes, enhance teachers' pedagogical content knowledge, and change teachers' attitudes towards and beliefs about professional development. An explanatory framework for learning study drawing on previous work of Guskey ("Evaluating professional development," Corwin, Thousand Oaks, CA, 2002) and Desimone ("Educ Res" 38:181-199, 2009) is proposed. These promising results suggest that schools should consider incorporating learning study as a regular and sustained practice among teachers. At the same time, teacher educators may capitalise on this form of teacher learning to refine the existing mode of teacher professional development.