Objective: This study was aimed at evaluating the effect of group metacognitive therapy on the modification of dysfunctional attitudes in male addicts under methadone treatment. Method: The number of 30 participants was selected from among the clients referring to New Tarighat methadone treatment center in Tehran in 2014 by purposive sampling and they were then randomly divided into two groups, i.e. experimental (n = 15) and control (n = 15) ones. The participants responded to Weissman & Beck's Dysfunctional Attitude Scale before and after receiving Metacognitive intervention. Results: The results showed that there was a significant difference between the two groups in dysfunctional attitudes in such a way that the experimental group received a lower mean score in dysfunctional attitudes. Conclusion: It seems that this training can be effective in preventing drug use slips and mistakes in addicts given to concepts such as acceptance, awareness-raising, presence at the moment, observation without judgment, confrontation, and release.