The purpose of this study is to investigate the relationship between the development of laterality and bilateral motor abilities measured with respect to various rhythm movements in upper extremity of mentally retared children. The development of laterality was measured by a survey of dominace-hand. The bilateral motor abilities were assessed by means of a tentative rhythm beat test of hands. One hundred sixty eighty subjects with special education needs shose age ranged from 7 to 18 years old were used for an experimental group. Fity nine normal children whose age ranged from 3 to 6 years old were also chosen as a control group. The major findings were a follows: 1. The development of lateral dominace-hand in mentally retared children differed from that of normal subjects with as increase in chronological age; the degree of the attainment of lateral dominance - hand was 75 percest of the mean level of the normal children. Though normal children establish a dominance - hand by ths age of years old, meatally retarded children are likely to show 57 percent of normal development at the age of 7 through 9 years old and 65 percent of normal development at the age of 16 through 18 years old. 2. A greater delay in mentally retarded was found through a rhythm test which was development to assess bilateral motor abilities in upper extremity. A closser relationship betwin chronological age and the development of bilateral motor abilities in mentally retared children was found. 3. In view of these findings, it was indicated that bilateral motor abilities of the retarded and non-retarded children were significantly correlated to the acqusition of dominance-hand.