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Critical thinking is very important in the 21^st century so that it should be improved by doing efforts such as making a lesson that train students to think independently and solve mathematics problems by their own. The purpose of this research is to describe Lesson Study activity and its roles in the development of mathematics critical thinking based on discovery learning. This study is narrative study of teacher’s experience in joining Lesson Study activity. This research used qualitative method design. The data collection was done by using three method such as observation, deep interview and documentations. The collected data will be analyzed with Milles and Huberman’s model that consists of reduction, display, and verification. The results show that thourgh Lesson Study activity, teachers know about how student mathematic critical thinking. Teachers also can revise their mathematics learning instrument in the form of lesson plan based on discovery learning model to improve student mathematics critical thinking. It means that discovery learning model with Lesson Study activity is important to make a better learning instrument and focus on how student learn.

Lesson Study, Mathematics Critical Thinking, Discovery Learning