Sandtray therapy, a modality of play therapy, has been used in a variety of ways as the treatment intervention with different theoretical approaches; however, there is a very limited amount of empirical research. The purpose of this research is to examine the effectiveness of group sandtray therapy at school with preadolescents identified with behavioral difficulties. This is a pretest-posttest control group design. Participants in the experimental group received sandtray therapy in group for ten weeks, and participants in the wait-list control group received no treatment intervention. The researcher compared two groups to examine the overall effectiveness of sandtray therapy as determined by the scores of the Child Behavior Checklist-Teacher Report Form (CBC-TRF), Parent Report Form (BASC-PRF), and Self Report of Personality (BASC-SRP). Based on teachers' reports, statistically significant difference existed between the two groups in terms of preadolescents' overall behaviors, externalizing behavior problems, and internalizing behavior problems after the ten week treatment intervention. The effect sizes were medium (d= .52-.59). According to parents' reports, a statistically significant difference was found regarding preadolescents' externalizing behavior problems, and the effect size was medium (d=.63). No statistically significant differences were found regarding preadolescents' total behaviors and internalizing behavior problems based on BASC-PRF. The effect sizes arranged from medium to small (d=.55 and .35, respectively). In terms of the total behavior on BASE-SRP, no statistical significant difference was found and the effect was small (d=.18). A case example was included to illustrate the process and effect of group sandtray therapy. Based on the results of this study, it is determined that group sandtray can be an effective treatment intervention for preadolescents identified with behavioral problems. The primary contribution of this study is to present empirical support for the effectiveness of using sandtray therapy.