The purpose of this study was to investigate the inhibitory effect of kaempferol on triple negative breast cancer cell MDA-MB-231 and its possible molecular mechanism.Cells were treated with kaempferol at different concentrations.CCK-8 method was used to detect the effect of kaempferol on the viability of MDA-MB-231 and human normal mammary epithelial cell MCF-10A.The morphological changes of cells in each group were observed by inverted microscope.Clone formation method was used to detect cell proliferation.Cell apoptosis was detected by flow cytometry.The changes of mitochondrial membrane potential were detected by JC-1 staining.Western blot was used to detect the protein expression of B-cell lymphoma-2(Bcl-2),Bcl-2 associated X protein(Bax),cytochrome C(Cyt C) and Cyclin D1.Cysteinyl aspartate-specific protease 3(Caspase-3) and cysteinyl aspartate-specific protease 9 (Caspase-9) activities were detected by colorimetry.The results showed that kaempferol significantly inhibited the proliferation of MDA-MB-231 cells in vitro,induced apoptosis,and had no effect on the viability of normal mammary epithelial cells.It decreased the membrane potential of mitochondria,increased the expression of Bax and Cyt C,decreased the expression of Bcl-2 and Cyclin D1,and enhanced the activities of Caspase-3 and Caspase-9.It is suggested that the activation of mitochondrial apoptosis signal pathway is one of the pathways of kaempferol induced apoptosis in MDA-MB-231 cell.