The aim of the study was to test the effect of teachers' professional development upon students' mathematics academic achievement levels with the help of lesson study. In this manner, the study aimed at increasing students' mathematics learning levels via lesson study. Action research was used as research method in the study. The sample of the study was consisted of 32 6th- grade students who were studying at the school having lower academic success. Four different learning outcomes about "natural numbers activities" were tried to develop in the process of implementation of the study. The achievement test called as "the achievement test about natural numbers activities" was used as data collection instrument which was developed by the researchers. According to the findings of the study, there was significant difference between academic successes of the students. Lesson study is designed to support students' learning by choosing a lesson, a subject and an aim. According to the study's findings and results, it was seen that the lesson plan, research lesson, new teaching strategies and materials, which were prepared with different views altogether, made the students' learning levels and qualities increase and also develop reasoning.