The present study aimes to investigate the challenges that primary school teachers face regarding the lesson study. To this purpose, qualitative approach and case study were used as the method in this study. In the sampling phase, the mixed variation sampling was applied in two levels. In first level, the study population was all primary schools in Hamedan which included lesson study and maximum variation sampling was used to select the case for this study. The selected case was a primary school in the city margin of Hamedan which included male and female teachers with 2 to 27 years of experiences. In the second level, research population was all the educational staff of that school (n=20) and the sample selected was equal to the population (n=20) through comprehensive sampling. These individuals were studied since the announcement of the festival's lesson study program in the fall of the academic year of 2016-17 to the publication of the results of the festival. The research tool for collecting data were semi-structured interviews with teachers and participatory observation. For data analysis, coding and categorization were used. The results showed that teachers faced three categories of challenges. Challenges before practical engagement were:  knowledge in the field of study, motivation to participate in lesson study, the formation of a group and the assignment of duties of members.  At the stage of the practical encountering, the challenges were: the choice of the problem for the study course, the implementation of the lesson and the collection of data, the review of teaching. After the practical encountering with lesson study, challenges were: the arbitration of study course plans, the publication of the results of the study course, the holding of the festival of research. In the end, a model was proposed with the purpose of reducing these challenges.