The purpose of this study is to comprehensively present the effects of group music therapy for children with developmental disabilities and to determine the total effect size. To that end, we calculated 33 effect sizes in 14 studies consisting of domestic theses and journal articles published in recent 20 years (1998~2017). The findings of this study are as follows. First, the group music therapy for children with developmental disabilities was effective. The total mean effect size analyzed with a random effect model turned out to be a medium effect size of .761. Second, the mean effect size by object group was the largest in children with autistic disabilities, followed by children with developmental disabilities and children with intellectual disabilities. The mean effect size by age group was the largest in adolescents, followed by preschool children, and school-age children. Third, the mean effect size by intervention method was the largest in music and movement, followed by re-creative playing, singing, and improvisation, and all the effect sizes were medium level. Fourth, the mean effect size by intervention purpose was the largest in social area, followed by language area and emotion area, and all the effect sizes were medium level. Fifth, by the number of sessions, the group music therapy showed an effect size in 1~10 sessions and 11~20 sessions. The results of this study are meaningful as basic data for the presentation of future directions and the program development of group music therapy research for children with developmental disabilities.