Mentally retarded children mean that subnormal development, manifested from birth or early childhood, that may be caused by a variety of conditions so that they are insufficient the mental faculties. They are very difficult to deal with their life circumference and to adapt themselves to social life. But music has more powerful and emotional effect than the other arts in respect of the music functions directly without any concrete concept or the mediation of language. In this respect, this study observes the effect of music education on mentally retarded children through the documents and proceding study. Then, this study intends of spread widely the educational application of music in our real education for the retarded. I will convey the following results from this study. First, the mentally retarded children who have difficulty in verbal conversation, could improve the difficulties of communication through the non-verbal activities. And these activities have the effect upon the subconscious music world and bring about the will to live and the self-confidence. Second, the bulk of the mentally retarded children have the speech defect remarkably but the singing study effective for them. Usually, they are not stammer when they sing a song so that singing is effective to subjugate, their speech defect that to pronounce with piano or other instruments accompaniment and to speak as modulated with the musical scale. And the direction of the chours is helpful to increase the agreement of group as many people sing together. Third, the bulk of severe mentally retarded children have a troublesome of physical expression because of their discomfort limbs. The rhythmic movement and instrumental music study could improve their physical development and flexibility and help the better application of their discomfort limbs. Fourth, in spite of their diffuseness, musical activity could promote their attentiveness and rhythmic movement could relax their tensions and impressive activity could bring a emotional sense of security to an unsecure feeling. These factors are effective over the whole life of mentally retarded children.