This study aimed to improve mathematics teachers’ formative assessment practice in junior high schools through a teacher professional learning community. Teachers learned to integrate instruction and assessment in a lesson in which they examined students’ learning progress by questioning and then gave students immediate feedback accordingly to push students learning forward. In addition, this study examined whether or not students’ learning achievement in mathematics benefits from the improvement of teachers’ instruction. The researchers applied Mehan’s (1979) theoretical framework to analyze teacher-student discourse in mathematics classrooms and to inspect the progress of participating teachers’ questioning/feedback. Students’ mathematics learning achievement in the treatment group was compared with that of students in the control group in terms of one-way ANOVA. Results suggested that: (1) In the beginning, the participating teachers tended to use low-level questions to conduct teacher-students discourse and did not allow students enough time to answer the questions; (2) While the teacher professional learning community was operated in terms of a three-stages and cyclic model, teachers’ formative assessment practice was improved effectively. Participating teachers began to probe and to clarify students thinking by posing high-level questions with an aim to guide students learning step-by-step; (3) Students’ mathematics learning achievement benefited from the development of their teacher’s teaching. This study also reported the influence of the learning community on the participating teachers’ personal professional learning and provided suggestions for teacher instruction as well as suggestions for subsequent studies.