The purpose of this study was to examine the effects of forest activities on the movement skills, fine muscle functions and prosocial behaviors of children with a developmental disability. 12 children, aged 8 to 10, participated in this study; all of whom were either autistic or had an intellectual disability. The participants were divided in half and designated to either the experimental group or to the control group. The Forest Activity program was carried out once a week (from 10 a.m. to 12 p.m) for 10 weeks in a forest near D school in Seoul. Gross Motor Skill (MSTB-B), Fine Motor Skill, and prosocial behavior (PBSYC) was used as measuring tools to test the effects of the Forest Activity program on the participants. The application of the Forest Activity program showed improvements in flexibility and equivalence areas of Gross Motor Skill; as well as improved fine motor skills in accuracy and stability areas of small muscle function; and finally, there was an increase of prosocial behavior in 5 areas out of a total 7 areas (leadership, providing assistance, communication, empathy, proximity seeking). This study supports previous research in demonstrating the effectiveness of programs applying forest activities, such as the forest healing program, to enhance physical and mental stability.