Background: The purpose of this study was to present a practical method of medical treatment to improve the balance and motor ability of the mentally retarded child with a single mentally retarded child-subject. Methods: The subject of the study was a 39-month-old mentally retarded female. This study included a 2-week basic period and a 13-week treatment period. The treatment method was based on the Bobath Approach. Gross motor function measurement (GMFM) was used to examine changes in motor ability, and the Pediatric Balance Scale (PBS) was used to measure changes in balance ability. The curative program was composed of normalization of muscle tone, strengthening of leg endurance and muscular strength, the improvement of trunk alignment, and the increase of balance. Visual rate of change was used to examine the results. Results: As a result of this study, balance ability increased on the Pediatric Balance Scale (PBS) by 24 points, and motor function increased in terms of Gross Motor Function Measurement (GMFM) by 6.9% (18 points). Standing increased by 41% (16 points), and walking, running, and jumping increased by 31.9% (23 points) compared to thebasic period. Therefore, the Bobath Approach appears to be an appropriate method to improve balance and motor ability in mentally retarded children. Conclusion: It is surmised that aggressive intervention by physical therapists and occupational therapists, and a follow-up study, are required for the growth of motor ability in mentally retarded children.