Tanshinone I (Tan-I) and tanshinone IIA (Tan-IIA) were isolated from Danshen (Salviae Miltiorrhizae Radix), a widely prescribed traditional herbal medicine that is used to treat cardiovascular and dysmenorrhea diseases. In our previous study, Tan-IIA was demonstrated to induce apoptosis in human colon cancer Colo 205 cells. However, the effect of Tan-I on human colon cancer cells is not clearly understood yet. In this study, the anti-growth and apoptosis-eliciting effects of Tan-I, as well as its cellular mechanisms of actions, were investigated in Colo 205 human colon cancer cells. Tan-I reduced cell growth in a concentration-dependent manner, inducing apoptosis accompanied by an increase in TUNEL staining and in cells in the sub-G1 fraction. The expression of p53, p21, bax and caspase-3 increased in Tan-I-treated cells. In addition, the cell cycle analysis showed G0/G1 arrest. These findings suggest that Tan-I induces apoptosis in Colo 205 cells through both mitochondrial-mediated intrinsic cell-death pathways and p21-mediated G0/G1cell cycle arrest. Accordingly, the therapeutic potential of Tan-I for colon cancer deserves further study.