Abstract: 47 cases of Mental Retartion （MR） were studied for the the cause, basing on brain CT scan and immunological functions. It was found that the perinatal factor and infections appear to be most common of the various causes. Lower density in the brain lobi and encephalatrophy were seen by CT scanning. Thus, CT scan may be helpful to the clinical diagnosis. The changes of immunological functions were also found in children with MR.