Lesson Study was a teaching professional cycle which the teachers worked together. As a result, the teachers better understood the mathematics content and teaching as well as the students’ thinking (Lewis, 2002). Most of Lesson Study included multi stages of practice, and work relating to research (Fernanadez & Yoshida, 2004). In Thailand, Assistant Professor Dr. Maitree Inprasitha brought the model and method from Japan by adjusting to Thai classroom culture into cycle of Les- son Study to emphasize the teaching profession in school by bringing major principles to adjust into only 3 phases as: 1) Collaboratively design research lesson (plan)—term consists of teachers, researchers, school-coordinator, and coach formulated open-ended problem; 2) Collaboratively observing the research lessons (do) bringing lesson plan from 1) to implement teaching in classroom including the teacher as a group member to teach, all of members had objective in observing the learning management as: to observe the students’ thinking method using in participating in activities of problem solving the open-ended problems,; and 3) Collaboratively Reflection or Post-discussion (see), every member from 1) discussed after teaching in order to improve the lesson plan together. The important part of problem situation development as open-ended problems, Assistant Professor Dr. Maitree Inprasitha integrated the Lesson Study with mathematics teaching model focusing on the Open Approach (Inprasitha, 2006).