In this article, an in-service training program was developed and evaluated to improve the peace education competencies of primary school teachers. This program, named as In-Service Training Program with the Theme of Peace Education for Primary Teachers (BEHEP), was based on the system approach. The implementation was completed in 28 hours with the participation of 18 primary school teachers working in Serkan Ciddioglu Primary School located in Melikgazi district of Kayseri province. The article was designed according to the mixed research model. Interview forms, concept forms, course plans, participant diaries and in-service training evaluation scales were used as data collection tools. Qualitative data were analyzed via content analysis method; quantitative data were analyzed via SPSS. BEHEP has been an effective program to raise the awareness of teachers and to improve their skills in designing learning environments suitable for peace education.