Abstract: Diabetes mellitus increases the risk of breast cancer and affects the prognosis of breast cancer patients.Metformin, an anti-diabetic drug, decreases blood glucose level and has a role in suppressing various cancers.In addition, metformin has a unique inhibitory effect on breast cancer, i.e., it can inhibit breast cancer cells in vivo and in vitro.Moreover, metformin exerts its anti-tumor effect on epidermal growth factor receptor 2(HER-2) positive and monoclonal antibody trastuzumab-resistant breast cancer cell lines, breast cancer stem cells, and triple negative breast cancer cells.Metformin can reduce the risk of the breast cancer in patients with diabetes, lower histologic grade, and increase expression of estrogen and progesterone receptors.Furthermore, metformin has certain effect on neo-adjuvant chemotherapy for breast cancer.This review aims to clarify the mechanism of metformin in restraining breast cancer based on basic and clinical study results.