The goal of this study was to determine whether tetrandrine enhanced radiosensitization in different hepatocellular carcinoma cell lines and to elucidate the potential mechanism. We also tested whether PA28γ was regulated by tetrandrine. The human hepatocellular carcinoma cell lines HepG2 and LM3 were divided into six groups: control; low-dosage (0.5 or 5 μg/ml) tetrandrine alone; high-dosage (1.0 or 10 μg/ml) tetrandrine alone; irradiation alone; irradiation with low-dosage (0.5 μg/ml or 5 μg/ml) tetrandrine; and irradiation with high-dosage (1.0 μg/ml or 10 μg/ml) tetrandrine. Colony-forming assays were performed. Expression of cyclin and apoptosis-related proteins, including cyclin B1, phosphorylated cyclin-dependent kinase 1 [phospho-CDC2 (Tyr15)], Bax and caspase-3, as well as PA28γ expression, were evaluated using Western blot analysis. Apoptosis rate and cell cycle distribution were examined using flow cytometry analysis. Tetrandrine enhanced radiosensitivity in HepG2 and LM3 cells, as characterized by a narrower shoulder area and steeper linear area, and the enhanced radiosensitization increased with tetrandrine dosage. After tetrandrine treatment, the apoptosis rate significantly increased, whereas the proportion of cells in the G₂ phase dramatically decreased in dose- and time-dependent manners after irradiation. However, the effect of reverse G₂ arrest was weaker in p53-mutant cells (LM3 cells). Finally, we observed that tetrandrine downregulated PA28γ expression. Moreover, when PA28γ was downregulated, apoptosis and cell cycle distribution were also altered; however, the effects were weaker in p53-mutant cells. Therefore, we propose that tetrandrine-mediated apoptosis induction and G₂ arrest attenuation are at least partly mediated by PA28γ.