Amentoflavone, isolated from an ethyl acetate extract of the whole plant of Selaginella tamariscina, a traditional herb, may exhibit antitumor activity. The aim of this study was to investigate the anticancer mechanism(s) of amentoflavone, such as mitochondria-mediated apoptotic cell death, in typical breast cancer MCF-7 cells. Cells treated with amentoflavone exhibited a series of cellular alterations related to apoptosis, including DNA and nuclear fragmentation, and de-regulation of intracellular reactive oxygen species (ROS) and calcium. In addition, markers of mitochondrial-mediated apoptosis, including the reduction of mitochondrial inner-membrane potential, the release of cytochrome c from mitochondria, and activation of caspase 3, were observed. In conclusion, our results present, to our knowledge, the first evidence that amentoflavone induces apoptosis of MCF-7 breast cancer cells, and that this is closely related to mitochondrial dysfunction. Amentoflavone may be a potential therapeutic agent for breast cancer treatment.