The aim of this study was to investigate the effect of peer-mediated adaptive physical activity program on problem behaviors of mentally handicapped students. Eight mentally handicapped students and eight peer students with normal development participated in this study, which was designed according to a sequential descriptive design. In the research, peer education program was applied to the peer students in line with the skills expected to be exhibited during the practices. In the study, Problem Behavior Sub-Scale (PBS), which is one of the subscales of Social Skills Rating System (SSRS) developed by Gresham and Elliot (1990) and Turkish adaptation by Sucuoglu and Ozokcu (2005) was used as a quantitative data collection tool. As a qualitative data collection tool, semi-structured interview form was used. The Friedman test was used to analyze the quantitative data, and Wilcoxon Signs test was used to determine the source of the difference. Significance value was determined as 0.05. In the analysis of qualitative data, content analysis was applied. The quantitative findings of the study showed that the peer-mediated APA program was effective on the problem behaviors of mentally handicapped students and that the skills acquired as a result of the practice were maintained by the children three weeks after the completion of the practices. In the qualitative findings, the teachers stated that after the peer-mediated APA program, there were positive developments in the ability of mentally handicapped students to experience push-pull, touch discomfort, fear, trust, power control with their peers and to act in accordance with the instructions. The obtained qualitative and quantitative findings showed that peer mediated APA program had positive effects on the problem behaviors of mentally handicapped students.