Breast cancer patients need coping mechanisms to adapt to changes due to breast cancer. The constructive coping mechanism is very influential on the attitude, response, decision making towards breast cancer management, and quality of life. The study aimed to determine the determinants of coping mechanisms of breast cancer patients in Sidoarjo Regency, Indonesia. The sample size was 98 breast cancer patients. The variables analyzed included coping mechanism, age, educational, marital, work, time diagnosed, genetic, stage of cancer, therapy, social support, and spiritual need. The results found that breast cancer patients with higher education had a 51.200 times chance compared to breast cancer patients with no education to have adaptive coping mechanisms. Therapy for breast cancer patients has a positive effect on adaptive coping mechanisms. Social support has a positive influence on breast cancer patients to have an adaptive coping mechanism. The higher the social support provided, the more adaptive coping mechanism is for breast cancer patients. Tree variables have proven to be significant as determinants of coping mechanisms among breast cancer patients in the Sidoarjo Regency, namely education, therapy, and social support.