The purpose of this study was to investigate the effects of a roleplay based self-determination activity program on the self-determination skills and adaptive behaviors of high school students with mental retardation. One-group pretest-posttest design was used for this study. Ten male students with mental retardation participated in the 48-session program conducted for 12 weeks. The program included 48 role-play situations. Self-determination skills of subjects were measured by two special education teachers before and after implementing the program using the Program-based Self-determination Skills Scale developed by the author. The subareas of self-detemination skills were self-awareness, self-management skills, choice-making skills, self-advocacy skills, support networking skills, community use skills, social skills, cooperative skills, and stress management skills. Adaptive behaviors were measured by two special education teachers before and after implementing the program using the rating scale developed by Wasserman et al., (1979). Matched pair t-test was used for the analyses. The research findings were as follows: (1) There was a statistically significant improvement of self-determination skills of high school students with mental retardation after implementing the role-play based self-determination activity program. (2)There was a statistically significant improvement of all of 9 sub-areas such as self-awareness, self-management skills, choice-making skills, self-advocacy skills, support networking skills, community use skills, social skills, cooperative skills, and stress management skills. (3) There was a statistically significant improvement of adaptive behaviors of high school students with mental retardation after implementing the role-play based self-determination activity program. The implications of the research results were discussed.