Abstract Backgrounds: To assess patients' satisfaction and to evaluate methadone therapy program, it is important to understand the experiences of opium dependents during the treatment period in methadone centers and determine the quality of this program and revise standards based on that. This study aimed to describe the nature and structure of patients' experiences during treatment in methadone centers. Methods: This was a qualitative method using phenomenology. Sampling was purposive and the participants were selected from opium dependents referred to Kerman methadone centers during 2007. Sampling continued until data saturation and the sample size was 32. Colaizzi's method was applied for data analysis. Findings: The findings of this study included 27 codes categorized in four main groups: experiences of structure, personnel, patients, and regulations. These four categories showed the main structure of experiences in methadone centers. Conclusion: Lack of treatment centers in near-by cities or the problems of those centers suggest that it is necessary to establish new centers or solve the problems of centers in near-by cities. The type of patients referring to the centers plays a role in treatment process. The regular presence of physicians and other personnel and their concerns and care for patients as well as longer working hours of the centers have roles in patients' satisfaction and increase of consistency with treatment. Discussing the rules and regulations of the center with patients including the obligatory of daily reference to the center to obtain medicine and injections sound necessary. Also, it is necessary to find ways for solving problems of urine tests. Keywords:Experiences, Methadone, Performance, Qualitative study.