The purpose of this study was to investigate the effect of self-control focused group game play therapy on the improvement of peer relations of aggressive children. The research target is four fifth graders who have difficulty in peer relations due to their aggressive behavior at K elementary school in D city, and self-control focused group game play therapy was conducted a total of 10 times for five weeks, twice a week. The results of the study are as follows. First, it affected children s aggressiveness and sub-areas: physical aggression, verbal aggression, bypass aggression, negativity, excitement and hostility. Second, it influenced children s peer relationships and sub-areas: help, exclusivity, recognition, self-expression, and conflict resolution. Third, peer-relationship skills such as cooperation, self-assertion and self-control have increased within the session. Therefore, self-control focused group game play therapy can be verified as effective case study to improve peer relations of aggressive children.