Objective: The aim of this study was to investigate the effectiveness of therapy based on trans-theoretical model (TTM) in coping strategies among male substance dependent patients under methadone maintenance therapy in Najaf Abad city. Method: To this end, a quasi-experimental research design along with control group was employed the relevant evaluation was performed in pre-test, post-test, and follow-up stages. The statistical population of this study consisted of all the male patients under methadone maintenance therapy in Addiction Treatment Centers of Najaf Abad City within the summer of 2013. From among this population, the number of 30 participants was selected as the study sample via multi-stage random sampling method and these participants were then randomly assigned to experimental (n=15) and control(n=15) groups. The experimental group received 10 sessions of therapy based on trans-theoretical model within 2 months in addition to receiving methadone, whereas the control group only remained under methadone maintenance therapy. Coping Inventory of Stressful Situations was used in this study for data collection purposes. Results: The results obtained from covariance analysis showed that the therapy based on trans-theoretical model was effective in the improvement of coping strategies in the posttest (P < .001); however, these results did not show any significant difference in coping strategies scores in the follow-up stage. Conclusion: According to these results, it can be concluded that therapy based on trans-theoretical model can be used as an appropriate intervention in addiction treatment.