The purpose of this study is to find out whether a small group play therapy program changes children's externalizing behavioral problems. The subjects of the study were three children who had difficulties interacting due to externalization behavioral problems who had applied to participate in the 'small group play therapy' of the M center located in G city. A small-group play therapy program was conducted twice a week for a total of 10 sessions for 5 weeks. To find out the change in externalization behavioral problems, the externalization scale was measured and compared pre, post, and follow-up. In order to changes in children's externalization behavioral problems, changes in children's externalization behavioral problems within the group were measured each time during the 10 sessions of the small group play therapy program. The results of this study are as follows. First, small-group play therapy decreased children's K-CBCL externalization scale scores. Second, small group play therapy reduced the number of externalization behavioral problems (screaming, pushing, throwing objects) in children. Third, small group play therapy changed children's problem behaviors into positive behaviors. This study suggests that a small group play therapy program is useful in demonstrating positive behavioral changes in children with externalizing behavioral problems. However, as a small number of case studies with 3 children, there is a limit to generalize the results.