Background: Poor adherence of patients with type 2 diabetes to treatment is one of the most complex and important clinical concerns. It is the main issue of the present decade and acknowledged as a challenge to control and treat diabetes. This study was carried out to explore and understand how adherence to treatment process occurs among Iranian patients with type 2 diabetes.Methods: The present study is qualitative with grounded theory approach. The data were collected from December 2015 to July 2016 in Tehran (Iran) through individual semi-structured in-depth interviews, field notes, and memos from 21 patients with type 2 diabetes; combined with two members of their families and a healthcare professional. The data were analyzed based on Corbin and Strauss constant comparative analysis (2008).Results: Adherence to treatment is a transitional, interactive, and continuous process. For patients with diabetes, this process includes unperceived threat in diagnosis time (poor knowledge and skills, bottleneck of dependencies, superficial understanding of the new situation), bitter belief (downhill quality of life, physical and emotional treatment feedbacks), and adaptation to treatment (self-care dominance, regimen integration in daily activities). The process of adherence to treatment was influenced by knowledge and skill, social support, beliefs and values, psychological characteristics of people, and the nature of diabetes. Conclusion: Adherence to treatment in Iranian people with diabetes depends on the family and social context, which is challenging for the patient and leads to the negligence of health behaviors. It is vital for healthcare providers to identify these factors to encourage patients to adhere and commit to treatment in order to prevent irreversible complications of diabetes.