The purpose of this study was to examine the effect of role play on the morality of mentally retarded children, who lagged behind physically, intellectually and socially, and to explore how they could maintain smooth social relations with nondisabled children. It`s specifically meant to compare the degree of morality between nondisabled children and mentally retarded children who were of the same mental age, and to find out the effect of role play on the development of the morality of mentally retarded children. The findings of this study were as follows: First, there appeared no significant difference in moral judgment level between the nondisabled and mentally retarded children of the same mental age. Second, the role play exercised a positive influence on developing the morality of the mentally retarded children. This suggests that actual teaching experience is a vital factor to affect the growth of morality, though it had close relations to intellectual maturity. There are some suggestions in conjunction with the findings of this study: First, the mentally retarded students from middle school special classes were examined in this study, but the range of subject should be extended to kindergarteners and elementary schoolers. In addition, in which way nondisabled and disabled children could be engaged in role play together should be studied. Second, the subjects in this study had a mild mental disability, and if there are systematic and well-organized role-play programs tailored to the individual areas of disability and its extent, their educational effect will be greater.