In this study, it was aimed to investigate the effect of lesson study, a professional development model, on the perceptions of middle school mathematics teachers about subject area competencies and the views of the teachers about this model. In this study, which was conducted with a qualitative research method, the data were collected through semi-structured interviews from the middle school mathematics teachers after three lesson-research-cycles. The descriptive analysis of the data presented some findings: a) identification/determination of Mathematics Subject Area Competencies (MSAC) while preparing and applying a lesson plan, b) being able to use their knowledge of mathematics, c) developments of the students' mathematical skills through groupwork and d) a more active teaching process with Lesson Study (LS). On the other hand, it was also seen that there are some negative findings showing that preparing a detailed lesson plan is troublesome and the teachers have difficulty in coming together and using the technology. In this context, it is believed that this study will enlighten the researchers who are interested in LS method to learn about the subject area competencies of teachers from different branches, to learn about the effect of mathematics teachers' general competencies related to teaching profession on teaching and about teachers' technological competencies.