This study aimed to investigate the effects of self-expression group game play therapy on peer relationships in children with aggression. The study participants were eight male students in the 5th and 6th grades who scored more than 100 points in the aggression test and more than 65 points in the K-TRF in the U city local children's centers. They were assigned to either the experimental group (N=4) or the control group (N=4). Self-expression group game play therapy was conducted for 10 sessions to assess intimacy formation, self-expression behavior understanding, self-expression behavior training, and activity reflection. The results of this study were as follows: first, the peer relationships, as assessed by the children in the experimental group, improved; second, peer relationships, as assessed by teachers for the children in the experimental group, improved; third, the children in the experimental group exhibited increased prosocial behavior and decreased disruptive behavior in peer relationships during the session. Therefore, self-expression group game play therapy had a positive effect on peer-to-peer relationships and behavior changes in aggressive children.