This study used a non-equivalent control group pretest-posttest design which was conducted to investigate the effects of a reality therapy group treatment on the locus of control and mental health of juvenile delinquents. The research process of this study was as follows : subjects received an eight sessions reality therapy group treatment program over a four week period. The locus of control and mental health was measured with the internal-external locus of control scale developed by Rotter(1966) and translated by Cha(1973) and the symptom check list-90-revision developed by Derogatis(1976) and translated by Kim(1984) to investigate the effects of the reality therapy. The results that were not hypothesized were : there were no significant differences in the locus of control scores and mental health scores between the experimental group and the control group after treatment. But if the follow-up studies were taken, the results may have a possibly supported the hypothesis because the treated subjects` locuses of control moved in an internal direction and were reduced in anxiety, depression, phobic anxiety and somatization among the mental health status after treatment as compared to the four weeks prior to treatment.