OBJECTIVE：To study the effects of costunolide on the proliferation ，migration and apoptosis of breast cancer SK-BR-3 cells and its mechanism. METHODS ：SK-BR-3 cells in logarithmic growth period were collected and cultured with different concentrations （10，20，30，40，50 μmol/L）of costunolide for 24，48，72 h. Inhibitory rate of costunolide on cell proliferation was detected with CCK- 8. The cells were divided into blank control group and costunolide group （10，20，30 μmol/L）. Hoechst 33258 fluorescence was used to observe the morphology and apoptosis of cells ，and apoptotic rate of cells were calculated. Cell scratch test was used to detect the migration ability of cells and calculate the migration rate. Western blotting was used to detect the relative expression level of Bcl- 2，Bax，Caspase-3 and Cleaved Caspase- 3 in cells. RESULTS ：The proliferation of SK-BR-3 cells were significantly inhibited by costunolide （P＜0.05 or P＜0.01），and it shows a trend of concentration and time dependence. In the blank control group ，cells possessed clear contour ，regular shape and good adherence . Compared with blank control group，the number of cells were decreased significantly in 10，20，30 μmol/L costunolide groups，the cell structure was loose，the volume was reduced ，and the gap became larger ，and most of the cell contour disappeared and became round ，the cell adherence was poor ；cell migration rate and Bcl- 2 protein relative expression level were decreased significantly ，while apoptosis rate and the relative expression level of Bax ，Caspase-3 and Cleaved Caspase- 3 protein were significantly increased （P＜0.05 or P＜0.01）. CONCLUSIONS ： Costunolide can inhibit the proliferation and migration ，and induce apoptosis of human breast cancer SK-BR- 3 cells，mechanism of which may be through up-regulating the expression of Bax ，Caspase-3 and Cleaved Caspase- 3 while down-regulating the expression ofBcl-2.