Tetrandrine, isolated from the root of Stephania tetrandra, has been used in Chinese medicine for the treatment of silicosis and arthritis, and it also has anti-tumor/growth activities. However, the signaling pathways of tetrandrine-induced growth arrest and apoptosis in cancer cells remain unclear. We investigated the molecular mechanisms of tetrandrine-induced apoptosis and growth arrest in human lung carcinoma cells. Upon treatment with tetrandrine, a time-dependent inhibition of cell growth was observed and cells developed many of the hallmark features of apoptosis. Flow cytometry analysis confirmed that tetrandrine increased populations of both apoptotic sub-G1 and G1 phase. Tetrandrine-induced growth inhibition was associated with induction of Cdk inhibitor p21, inhibition of cyclin D1 and activation of caspase-3. Tetrandrine also affected the expression patterns of cytoskeletons including distribution of F-actin and expression level of microtubule. These results suggest that tetrandrine merits further investigation as a cell cycle blocker as well as a cancer chemopreventive agent.