Objective: This study aims to investigate the protective and sensitive effects of melatonin (MLT) in the treatment of breast cancer. Materials and Methods: ER+ breast cancer rat model was established and then rats were randomly divided into five different groups as follows: control group, Diss group, adriamycin (ADM) group, MLT group, and MLT combined with adriamycin (M+A) group. Tumor weights and one month survival rate were compared among these groups. In addition, changes of tumor tissues and expression of Ecadherin were observed under optical microscopy or electro-microscopy. Results: Tumor weights were significantly lighter in M+A group than those in ADM group (p < 0.05). Under optical and electro-microscopy, tumor cell apoptosis was obviously increased in MLT group, and tumor cell injury was more severe in M+A group than that in ADM group; additionally, expression of E-cadherin was higher in MLT group and M+A group than that in other groups. Moreover, MLT group had the highest one month survival rate (100%), there was the poorest life quality in ADM group, but the best life quality in MLT. Conclusion: MLT could enhance the sensitivity of tumor to ADM in vivo and improve patient’s life quality.