The purpose of this study is to determine whether a small-group play therapy program affects children's externalizing behavioral problems. The subjects included three children exhibited difficulties in interactions due to externalization behavioral problems and had applied to participate in the “small-group play therapy” at M center located in G city. This program was conducted twice a week in 10 sessions over 5 weeks. An externalization scale was used to measure and compare pre, post, and follow-up stages in children's externalization behavioral problems to identify the variations in the children's responses during the 10 sessions of the program. The results of this study are as follows. First, small-group play therapy reduced children's K-CBCL externalization scale scores. Second, the therapy reduced the number of externalization behavioral problems (screaming, pushing, throwing objects) among children. Third, children's problem behaviors became positive behaviors. This study suggests that the small-group play therapy program has positive effects in relation to the children’s externalizing behavioral problems. However, a small case study with 3 children, limits the results toward generalization.