The purpose of this study is to investigate the effects of group psychotherapy on reducing aggression of institutionalized children. For this study, the first grade 4 children who live in an institutional facility were chosen as the experiment group have been taken the group psychotherapy, each session takes 70 minutes, total 24 times. This study was designed to experiment a pre, post, and follow-up test. According to the result, the children in the experiment group showed statistically meaningful decrease from the control group in the aggression level and lower domains of verbal aggression, negativity, excitability, hostility. The aggression of the experiment group showed meaningful decrease perceived by the teacher, the aggressive factor in the HTP inspection also showed meaningful decrease. As stated above, this study revealed that group psychotherapy program was effective on reducing aggression of institutionalized children.