Objective:To investigate effects of berberine hydrochloride on breast cancer cells (MDA－MB－231) and related mechanism. Methods:Human breast cancer MDA－MB－231 cells were treated with different concentrations of berberine hydrochloride (0,10,25,50,75,100 μmol/L),then the proliferation rate of MDA－MB－231 cells was examined by MTT assay after 48 h and 72 h;the apoptosis rate of MDA－MB－231 cells was detected by staning with Annenxin V/PI EGFP and flow cytometry;the level of reactive oxygen species in MDA－MB－231 cells was detected by DCFH－DA staining;the mitochondrial membrane potential was detected by molecular probe JC－1;activity of Caspase3 was detected by Caspase3 Kit. Results:Berberine hydrochloride inhibited the proliferation of MDA－MB－231 cells in a concentration－dependent manner. It also significantly elevated the reactive oxygen species level,reduced mitochondrial membrane potential and activity of Caspase3 in the MDA－MB－231 cells. Conclusion:Berberine can inhibit the proliferation and induce apoptosis of MDA－MB－231 cells through mitochondrial－dependent pathway.