This paper aims to report a case study of exploring the effect of "assessment for learning" on improving student learning and facilitating teachers' professional development in the examination-oriented context of Hong Kong. By adopting Variation Theory of the Lesson Study approach, data were collected through pre- and post-tests, interviews with students and teacher participants and observation field-notes in order to help diagnose students' learning difficulties and provide evidence for teachers to refine their teaching strategies to enhance students' learning effectiveness. The students' improvement in learning performance informed the teacher participants of the usefulness of 'assessment for learning' in the classroom.