The purpose of this study is to examine the development of causal inference of mentally retarded children. The hypotheses set up in this study were as follows; First, when mentally age is controlled, there will be no differences in the nonverbal task performance between mentally retarded children and normal children. Second, mentally retarded children\'s nonverbal task performance will increase with the raise of mental and chronological age. Third, mentally retarded children\'s nonverbal task performance will be higher in familiar task than in unfamiliar task. Subjects in the were study consisted of 60 mental retarded children and normal children whose mental age was 4, 5, and 6-years old. The task used in this study was composed of 4 pictures that represent daily life event. Forty figure cards were composed of familiar and 5 set of unfamiliar figures which had familiarity- unfamiliarity types. In case of nonverbal task, children were asked to seriate the card in term of causal sequence. The results of this study were as follows; First, there was no difference in the performances of causal inference between normal children and mentally retarded children in the nonverbal task. Second, the performance level of caual inference in the mentally retarded children shows icrease with the chrnological age, where as no difference is revealed by mental age. Third, mental retarded children\'s nonverbal causal operational performance are better on familiar task higher than on unfamiliar task.