This study is to find the effect of filial play therapy for an infant with reactive attachment disorder. Subjects of this study were 3 years old boy and his mother. 2 therapists for play therapy with an infant and for parent education with his mother were involved. To collect datas about infant's abnormal behavior, interview with mother, CARS, K-DIAL, observation on infant, and questionnaires on maternal behaviors were tested. The result found the effect of filial play therapy for an infant with reactive attachment disorder was very useful.