The purposes of this study were to survey the actual conditions of balance disabilities in persons with mental retardation and to discuss their factors, from two aspects of dynamic balance evaluated by a beam walking test and static balance measured by a one foot balance test. Subjects were 157 persons with mental retardation aged 6 to 50 years. The relations between the performance of each balance and four variables (chronological age, mental age, clinical type and age of walking) were examined. Results were as follows: (1) Though balance abilities of persons with mental retardation were generally lower, two kinds of groups were found out. The first group showed lower performance in static balance while the other consisted of persons with lower ability in dynamic balance.(2) Individual age of walking was found to be closely related to both static and dynamic balance in all clinical groups.(3) Mental age was positively related to static balance without definite relation to dynamic balance. From (2) and (3), disorders of reflex system and behavior regulation were discussed as the factors in balance disabilities of persons with mental retardation.