This study aims to study the influence that the basic movement activity program for infants has on improvement of the physical expression ability of mentally retarded infants and on the physical expression ability according to the sex. For the study, 12 mentally retarded infants under the age of 6(6 male and 6 female infants) have been selected and examined. The physical expression ability of 6 infants had been examined for 30 minutes twice a week for 5 weeks before the test for the study. After the test for 5 weeks, the same test as the prior test has been made to check up the effect of the program. The result of the study is as follows: To begin with, the study shows that the mentally retarded infants who have participated in the program for 5 weeks shows better physical expression ability, to some extent, than those who have not. Accordingly, it is considered that the program of the basic movement activity has an effect on improvement of the physical expression ability of mentally retarded infants. And also, it is, the result of the study, not proved that the physical expression ability of the physically retarded male infants is different from that of the females. For the better result from c10se examination, it is considered that more infants should participate in the program and the session of the test should be prolonged. And also, it is, the result of the study, not proved that the physical expression ability of the physically retarded male infants is different from that of the females. For the better result from c10se examination, it is considered that more infants should participate in the program and the session of the test should be prolonged.