The aim of this study was to analyze the process for changing values about teaching mathematics for teachers in a pilot school implementing Lesson Study and Open Approach. The study was structured through a questionnaire survey of 83 teachers in 4 pilot schools. Case studies were then conducted with 3 of the teachers and involved participatory observations and video recording in 3 phases of Lesson Study, interviews and document analysis. Theoretically, the conceptualization of professional development with Lesson Study and Open Approach, values change (Rescher, 1969 cited in Seah, 2004) and change process (Fullan, 1985; Joyce & Showers, 1980) helps to explain the process for changing values about teaching mathematics. The study shows that teachers in a pilot school implementing Lesson Study and Open Approach have developed a new view and values about teaching mathematics (e.g. values in designing lesson plans, values in teaching practice, values in classroom assessment). The components were essential for changing teachers values about teaching mathematics and consisted of 1) Teachers participating and learning about the underlying theoretical principles of Lesson Study and Open Approach 2) Practicing weekly cycles of Lesson Study into school culture 3) Seeing Lesson Study and Open Approach demonstrated from expert and Japanese teachers 4) Ongoing coaching or support from the project and 5) Obtaining feedback from outsiders such as educators, parents, school board members etc.