This study aimed to find the effect of group music therapy on the self- expression of adolescent mental retardation. Mentally retarded adolescents showed the difficulty in acquirement in language, thinking and behavior regulatory function by retarded intelligence development. Adaptative behavior appeared to be defective by lack of achieving the task which is asked by the society. Consequently, it was important for them to express properly. This study examined to see the effect of self-expression through group music therapy for the mentally retarded adolescents who are able to be educated. With twenty four subjects who are attending the settlement after their school in J city, the experimental group ran 22 sessions of the group music therapy from November 6th 2006 to April 23rd 2007 for once a week. Followings were the result of the study. First, the group music therapy did not cause a significantly meaningful change in the verbal expression of youngsters with mental impediment. Second, the group music therapy caused a significantly meaningful change in the phonetic expression of youngsters with mental impediment. Third, the group music therapy appeared to have the positive effect on the nonverbal expression for the youngsters with mental impediment. From what we found it was revealed that the group music therapy had a positive effect on their self- expression. From the reality of lacking the systematic program, the result of this study was significant for suggesting the direction to help the adolescent with mental impediment to keep their life well by reducing the difficulty in proper self-expression.