The purpose of this study is to investigate the effects of a play therapy program for improving self-expression on the improvement of self-expression in non-disabled children that have siblings with developmental disabilities. The subjects of this study are non-disabled children between the ages of 8 and 10 who have siblings with developmental disabilities with low self-expression. The play therapy program to improve self-expression was conducted twice a week, 10 sessions of 60 minutes each. The results of this study are: first, the play therapy program for improving self-expression improved the self-expression scale score of the study subjects, and second, the frequency of self-expression behavior increased, and passive and aggressive behavior decreased. Though it is hard to generalize this study with a short-term program and few case studies, this study is significant in that it showed that the play therapy program for improving self-expression improved self-expression in non-disabled children that have siblings with developmental disabilities.