This study investigated the anticancer effects of thioflavanone and thioflavone in the MCF-7, MDA-MB-231 and MDA-MB-453 human breast cancer cell lines. Cells were treated with either thioflavanone or thioflavone from 1 to 100 µM for 24 h, and their anti-proliferative activity and cytotoxicity was determined. Thioflavanone and thioflavone possessed similar anti-proliferative activities; their IC50 values were 62-89 and 74-128 µM, respectively, although the cytotoxicity of thioflavanone was significantly higher and occurred in a dose-dependent manner. Taken together, these results suggest that thioflavanone significantly inhibits cellular proliferation with weak cytotoxicity to a greater extent than thioflavone, and induces apoptosis in human breast cancer cell lines. Moreover, thioflavanone, but not thioflavone, induces apoptosis via p53-dependent expression of Bax.