Abstract: Objective To study the inhibitory effect of tanshinoneⅡA on growth of human breast cancer(MDA-MB-231) cells with negative estrogen receptor(ER) and its mechanism. Methods Inhibitory effect of tanshinoneⅡA on growth of MDA-MB-231 cells with negative ER was detected by MTT assay and colony-forming unit assay.Morphology of MDA-MB-231 cells was observed under inverted microscope.Inhibitory effect of tanshinoneⅡA on cell cycle of MDA-MB-231 cells was detected by flow cytometry(FCM). Results The growth and colony formation of MDA-MB-231 cells decreased significantly in a time-and dose-dependent manner after tanshinoneⅡA treatment with its IC₅₀ of 0.125μg/ml(P<0.05).Typical changes of cell apoptosis morphology were observed under inverted microscope.The number of MDA-MB-231 cells increased at the G₀/G₁ phase and decreased at the S-and G₂/M-phase,demonstrating that tanshinoneⅡA can induce cell apoptosis. Conclusion TanshinoneⅡA exerts its antitumor effect by inhibiting the growth of MDA-MB-231 cells and inducing their apoptosis.