Objective: The current research was conducted to examine the effectiveness of stress coping skills training in the mental health of the addicts who referred to addiction treatment centers. Method: This research employed a quasi-experimental research design with pre-test and post-test. The statistical population of this study consisted of all clients in addiction treatment clinics who had registered for methadone treatment in 2014. The number of 30 participants was selected by convenience sampling method and divided into experimental and control groups. The experimental group participated in 8 sessions of stress coping skills training and, then, the mental health of both groups was examined by General Health Questionnaire. Results: The results showed that stress coping skills training has been effective in addicts’ mental health. Conclusion: Stress coping skills training can be effective in reducing the harmful effects of addiction on mental health.