Salvia miltiorrhiza (SM) is a traditional Chinese herbal medicine, commonly used to treat liver diseases in China for centuries. Several earlier studies have indicated that SM exhibits anti-tumor properties, but its mechanism remains to be elucidated. In this study, we evaluated the molecular mechanism of SM in a human hepatoma cell line, HepG(2). Our results show that SM exerted clear cytotoxic effects, and strongly inhibited the proliferation of HepG(2) cells. It was also observed that SM treatment caused apoptotic cell death as evaluated by: (a), morphological changes by using acridine orange/ethidium bromide staining; (b), DNA fragmentation by TdT-mediated dUTP nick end labeling (TUNEL); and (c), sub-G(1) cell analysis. Furthermore, depletion of intracellular glutathione (GSH) and reduction of mitochondrial membrane potential were found to be involved in the initiation of apoptosis by SM.