Background and Purpose:Aggressive behavior is the most frequent behavioral problem in the preschool children. Numerous factors play a role in this context, including the lack of social skills. In this regard, play is considered as one of the most efficient methods in reducing children's externalizing behavior problems and improving their communicative and social skills. Therefore, the present study was conducted to investigate the effectiveness of group play therapy on the aggressive behaviors and social skills of preschool children. Method: This study was a quasi-experimental research with pretest-posttest control group design. The study sample consisted of 24 preschool children with aggressive behaviors in Lahijan city over the academic year 2015-2016. They were selected by cluster sampling and then assigned randomly to either the experimental or control group. Preschool Aggression Scale (Vahedi et al., 2008) and Matson Evaluation of Social Skills (1983) were used for data collection. The experimental group received eight 45-minute sessions of play therapy (twice a week). One week after the end of the sessions, posttest was administrated to both groups. Analysis of covariance was used to analyze the data. Results: Findings showed that group play therapy was significantly effective in reducing aggression and its components like invasive-verbal aggression, invasive-physical aggression, relational aggression and impulsive aggression and in improving the appropriate social skills and selfconfidence (P=0.000). It also reduced significantly the inappropriate audacity, impulsiveness, jealousy and isolation (P=0.000). Conclusion: Based on the findings of this study, it can be concluded that play therapy gives this opportunity to the children to show their inner feelings caused by stress, frustration, fear, and confusion and to learn how to control or stop aggression. Since group play therapy can help the preschool children to learn skills like compliance with social rules, interact and collaborate with others, and management of negative emotions in interpersonal relationships; hence, using this method to prepare them for entering the broader social world seems necessary.Citation: Ebrahimi T, Aslipoor A, Khosrojavid M. The effect of group play therapy on aggressive behaviors and social skills in preschool children.