Nowadays herbal-derived medicines are attracting attention as new sources of drugs with few side effects.Silibinin is a flavonoid compound with chemotheraputic effects on different cancers such as examples in theprostate, lung, colon and breast. In the present study, the cytotoxic effects of silibinin on MCF7 breast cancercells were investigated. Apoptosis was determined by flow cytometry and the impact of silibinin on the expressionof pivotal genes including Bak, P53, P21, BRCA1, BCL-X1 and ATM was analyzed. Treatment for 24h had asignificant dose-dependent inhibitory effect on cell growth (p<0.05) with dose- and time- dependent inductionof apoptosis (p<0.05). In addition, there were significant increases in BRCA1, ATM, Bak and Bcl-XL geneexpression at the mRNA level with different concentrations of silibinin for 24 or 48 h (p<0.05). Taken together, theresults suggest that silibinin inhibits the proliferation and induces apoptosis of MCF-7 cells by down-regulatingBak, P53, P21, BRCA1, BCL-Xl and thus may be considered as an effective adjuvant drug to produce a betterchemopreventive response for the cancer therapy.