Background: Breast cancer is the most prevalent cancer among Iranian women and mastectomy comprises 81% of surgeries for treatment of breast cancer. Mastectomy may create feelings such as deformation or impairment in patients, cause body-image disorder, and reduce sexuality and sexual activity which in turn may entail mental disorders. The study aimed to elaborate coping processes. Materials and Methods: A grounded theory method was used in conducting this study. Twenty Iranian participants undergoing mastectomy were recruited with purposive sampling. An open, semi-structured questionnaire were developed. Obtaining consent, conversations were recorded and immediately transcribed after each session. Data analysis was carried out with the constant comparative method using the Strauss Corbin approach. Results: Analyzing the collected data, the study came up with seven main categories which affected the coping process in patients with breast cancer, namely: reactions to mastectomy; loss and death contest; reconstruction of evaluation system; consent for undergoing mastectomy; reactions and troubles after loss; confrontation of loss and health; and reorganization and compatibility with changes. Conclusions: The results of the study indicated: when patients become informed of their breast cancer and the necessity of undergoing mastectomy as the treatment, they probably pass through seven categories to adapt after mastectomy. Having insight about them is likely to contribute medical personnel in leading patients to the highest degree of feeling healthy.