The objective of this study is to examine the effect of musical rhythm on the physical rehabilitation of mentally retarded persons. To this end, this study sets the following research hypotheses: First, musical rhythm helps a client's body relaxation. Second, musical rhythm contributes to a client's gross motor development. Third, musical rhythm enhances a client's fine motor development. The object's musical responses were observed and analyzed at every term. Interviews were conducted before and after the experiment to examine the significance of the changes in physical rehabilitation that the object experienced in the musical rhythm therapy. The client was 34 years of age; he was diagnosed as mental retardation grade 2; as his physical movement, especially of the right hand, was unnatural because of the hemiplegia, experimental improvisation for body relaxation was conducted too as a type of dance therapy. The client was drawing pictures at an institution and participating in a church choral group. The findings of this study are as follows: First, musical rhythm helped play musical instruments and develop rhythmic senses. Second, musical rhythm helped body relaxation. Third, musical rhythm improved sociality through interactions and cooperation. In the interview conducted after the program ended, the object stated that the physical rehabilitation resulted in effective reactions. The findings above demonstrate that the treatment program based on musical rhythm was effective on physical rehabilitation of a person with physical disabilities. Thus, this study shows that the treatment program based on musical rhythm can be utilized as a therapeutic means that brings in positive effects on physical rehabilitation of those with physical disabilities.