This study aims to examine the body composition of MR children whose activity is significantly constrained compared with that of normal ones. To investigate the body composition of MR children, 119 normal boys, 105 normal girls, 198 MR boys and 109 MR girls were chosen. With their height, weight, %fat, fat weight, lean body mass and total body water, I reached to the conclusion as follows : 1. There are statistically significant differences in all factors except for fat weight in boys between the normal children and the MR children. 2. There are statistically significant differences in all factors between the girls and boys. 3. In both the boys groups, all factors were Showed significant differences statistically among the normal, EMR, TMR and DS children groups. 4. In both the girls groups, it appears statistically significant differences in all factors but one among the normal, EMR, TMR and DS children groups. In items of total 5. In each age(from 13 to 19 years old), it appears statistically significant differences in all factors between the normal children and the MR children in both the boys groups and the girls groups.