Lesson Study (LS) has been rapidly spread in many countries. It is a form of longterm professional development that teachers collaboratively plan, do research, and study on how to teach lessons to enhance students’ learning capability. This study aimed 1) to find out the perception of principals and teachers in the Islamic private schools towards LS, 2) to find out conditions and problems on the implementation of LS of the teachers in the schools. The samples were three school principals and seven teachers affiliated with Assalam Smart School Association. The qualitative data collection and the analysis employed were the indepth interviews, and content analysis, respectively. The study found that the principals and teachers perceived LS as a new significant approach to support teacher professional development. It was also discovered that the Islamic private schools began to practice it two years ago in the mathematics classrooms. The implemented lesson study consisted of seven processes as follows: 1) setting learning goals and designing problematic situations; 2) Islamizing situational knowledge; 3) creating teaching and learning materials, media and lessons based on the different approaches of thinking; 4) implementing the lesson plans; 5) classroom observation; 6) reflecting the results of teaching; and 7) exchanging knowledge among the teachers. It was explored the four major problems of its implementation namely 1) the ability to coach teachers, 2) lack of the principal involvement and support, 3) time management of LS team, as well as 4) lack of supporting facilities and resources.