The purpose of this study is to test our hypothesis that muscle relaxation is effective on the psychological well-being of juvenile delinquents. Subjects were 16 juvenile delinquents who had entered a reform school. Subjects were divided into two groups. The muscle relaxation group received muscle relaxation therapy once a week for a total of 4 times. The control group spent an ordinary daily life in the reformatory. Psychological questionnaires used were the Rosenzweig Picture Frustration Study (P-F study), the Psychological Stress Response Scale (PSRS), and the Eysenck Personality Questionnaire (EPQ). There was a significant Group Time interaction of the Group Conformity Rating (GCR) of the P-F study (F [1,14] =10.1, P=0.007). There were no significant interactions in the other psychological subscales. Thus, muscle relaxation therapy may improve frustration tolerance among juvenile delinquents.