Curcumin has shown therapeutic and/or adjuvant therapeutic effects on the treatment of some patients with breast cancer. However, its mechanisms of action are largely unknown. This study was designed to investigate its antitumor effect and underlying mechanisms in human breast cancer MDA-MB-231 and MCF-7 cells. The MTT assay was used to evaluate cell viability, and flow cytometry, acridine orange staining and transmission electron microscopy were used to detect apoptosis for cultured cells. The protein expression in cells was evaluated by western blot analysis. Breast tumors were established by subcutaneous injection of MDA-MB-231 cells in nude BALB/c mice, and curcumin was administered to the mice. The size of tumors was monitored and the weight of tumors was examined. The exposure of breast cancer cells to curcumin resulted in growth inhibition and the induction of apoptosis in a dose-dependent manner. We also found that the expression of Bcl-2 protein decreased and the expression of Bax protein increased which lead to an increase of the Bax/Bcl-2 ratio. In mice bearing MDA-MB-231 xenograft tumors, administration of curcumin showed a significant decrease of tumor volumes and tumor weight compared with the control. Our results showed that curcumin exhibited antitumor effects in breast cancer cells with an induction of apoptosis.