Leptin is a well-known mediator of obesity. Leptin and its receptor are overexpressed in breast cancer, especially in high-grade tumors. It has an association with progression and poor survival of breast cancer. Leptin can regulate endothelial cell proliferation and promote angiogenesis. There are several other factors such as insulin and HER2 may be involved in the relationship between leptin and breast cancer. Leptin system has emerged as a new and promising therapeutic target for breast cancer. This review article summarizes the current knowledge about the relation of leptin and breast cancer.