Background: Behavioral disorders create many personal and social problems. Children with such disorders encounter families and the community with various issues and difficulties, and make them vulnerable to psychosocial disorders in their adolescence and even long into their adulthood. The purpose of this study was to investigate the effect of play therapy on reducing the behavioral problems of preschool boys. Method: For this purpose, 20 children aged 3 to 6 years with behavioral problems were selected through available sampling, and were next randomly divided into two 10-people groups of experimental and control groups. The tool used in this research was Rutter's Child Behavioral Questionnaire (Form B). Participants were asked to participate in 12 play therapy sessions. Data were analyzed by covariance test. Results: The findings revealed that the mean scores of behavioral problems in the experimental group had a significant decrease in the post-test stage compared to the control group (P <0.05). The findings of the study indicate the effectiveness of group play therapy on reducing the behavioral complications of preschool children. This decrease was evident in both the total score and the subscales of children's Rutter behavioral questionnaire (aggression and hyperactivity, anxiety and depression, social incompatibility, antisocial behaviors, attention deficit). Conclusion: Given the reduction of behavioral problems in preschool children, the role of group plays as one of the strategies for treatment of behavioral disorders in children should be considered.