Objectives: The present study has done with purpose of survey the reasons of methadone maintenance therapy drop out in clients of Iranian National Center for Addiction Studies (INCAS). Method: In a qualitative study, sixteen clients underwent face-to-face deep semistructured interviews which continued until informational saturation. The interviews were written and studied by inductive-deductive content analysis method. Results: According to the analyzed data, treatment system, the patients, and the family are three effective factors in dropping out and bands of these factors influence each other. The patient encounters with issues including low motivation, unstable motivation, and comorbid psychiatric disorders. On the other hand, the family is not conversant about technique’s treatment, therefore don’t cooperate adequately. Also treatment system regarding many aspects (such as protocol inaccuracy, labeling, contemptuous behavior with client, and undertaking behavior with patient for example seeing him as a wrongdoer person, not to inform him, lack of enough appointment and empathy with him) is unaffordable for client. In this system, quantity is more emphasized than quality and psychological treatment, is too colorless. Conclusion: It seems that the current system and clinic`s conduct ways have been desined for patients with high motivation and severe dependency and it is not useful enough for patients with low motivation and moderate dependency. Hence these patients are not taken care of in the treatment program and drop out.