Purpose: The present study was conducted in order to compare the efficiency of individual and group cognitive-behavioral intervention in reducing psychological symptoms of mental disorders in male prisoners. Method: 48 prisoner subjects were selected and randomly placed in three groups of equal sizes: 16 received psychotherapy sessions which used the individual cognitive-behavioral method combined with cognitive-behavioral group psycho-education (combined therapy group), 16 only attended individual cognitive-behavioral psychotherapy sessions (individual therapy group) and 16 were placed in a group waiting to receive individual therapy (the control group). In order to identify the subjects' mental state, SCL-90-R and GHQ were administered both before and after the intervention, and the subjects were examined by psychiatrists and diagnosed according to DSM-IV-TR. The data were later analyzed using MANOVA. Results: The results showed that both of the therapy methods were effective, yet the combined method was more efficient. The subjects in both of the treated groups showed a decrease in the indexes of SCL-90-R and GHQ after the intervention, as compared with those in the control group. Follow-up of the released prisoners of the three groups showed that none of the subjects in the combined and individual therapy groups returned to prison within a period of one year, whereas 15% of the subjects in the control group did and were arrested and returned to the prison again in the same period of time. Discussion: The present study has a number of theoretical and practical implications. One is that giving insight to people, changing their attitudes and improving their adjustment skills may reduce maladjusted behaviors, a point already emphasized in various theories of psychology and psychotherapy. Moreover, the results remind specialists and authorities of the need for establishing psychotherapy clinics and providing individual and group psychotherapy to prisons and prisoners. (PsycINFO Database Record (c) 2016 APA, all rights reserved)