Tanshinone A is a novel derivative of phenanthrene-quinone extracted from Salvia miltiorrhiza BUNGE, a traditional herbal medicine. Cytotoxic effect of tanshinone A was observed in this study. Additionally its mechanism of promoting apoptosis was also investigated. MTT and SRB assays were applied to measure the effects of tanshinone A on the cell viability, the cell cycle distribution and cell apoptosis were measured by flow cytometry using PI staining and Annexin V/PI double staining method respectively. The changes of mitochondrial membrane potential were also detected by flow cytometry. Spectrophotometric method was used to detect the changes of caspase-3 activity. Western blotting assay was used to evaluate the expression of bcl-2, bax and c-Myc proteins. Results indicated that tanshinone A displayed a significant inhibitory effect on the growth of K562 cells in a dose- and time-dependent manner, and showed obvious minor damage to LO2 cells. Tanshinone A could arrest K562 cells in the G(0)/G(1) phase and induce apoptosis, decrease the mitochondrial transmembrane potential, decrease the expressions of bcl-2 and c-Myc proteins, increase the expression of bax protein and the activity of caspase-3. Accordingly, it was presumed that the apoptosis induction may be through the endogenous pathway. Subsequently, tanshinone A could be a promising candidate in the development of a novel antitumor agent.