Introduction: The effectiveness of child-centered play therapy on decreasing the symptoms of externalizing disorders in preschool children was investigated in this study. Method: Out of 120 preschool children, 10 children who had received the highest scores in externalizing scale in child behavior checklist (CBCL) were selected as the sample group. Teachers filled out the teacher report form (TRF) for each child. Members of sample group received 16 play therapy sessions in two member groups for eight weeks. Mothers completed CBCL forms after forth, eighth, twelfth and sixteenth sessions. At the end, teachers completed the TRF again. Results: Repeated measures analysis revealed a significant decrease in overall externalizing and aggressive behaviors scores. Comparing means of TRF scores in pre-test and post-test indicated a significant decrease in externalizing behaviors in the kindergarten. Means differences of first and second post-test as well as third and forth post-test scores were significant in externalizing scale and both aggressive behaviors and breaking rules subscales. Conclusion: Child-centered play therapy is an effective therapy for externalizing problems and this effect is more obvious for aggressive behaviors. Additionally, most remedy is occurred between first sessions and last sessions and the process was not salient in middle sessions.