Abstract: The aim of this study was to investigate the effect of ganoderic acid A (GAA) on the radiosensitivity of human hepatoma cell line HepG2 for high-LET neutron and low-LET gamma-ray radiations. In this research, the CCK-8 method was used to detect the inhibition of proliferation of HepG2 cells treated by GAA. It was found that 5 μmol/L GAA treatment had weak proliferation inhibition effect on HepG2 cells, and so this concentration of GAA was applied to the cells for 24 h in the subsequent experiments. CCK8 assay, clone formation assay, apoptosis assay, and the foci formation of \gamma H2AX were detected after irradiated by different doses of either neutron or \gamma -ray. The experiments showed that the high-LET neutron irradiation had a higher proportion of apoptosis than the low-LET γ ray irradiation for the cells without GAA treatment, while for the cells with GAA treatment, the proportion of apoptosis also decreased significantly for the cells under \gamma -ray irradiation. After treated with GAA, the proportion of induced apoptosis was significantly increased compared to the control group without GAA treatment. Besides, after GAA treatment, the cell proliferation inhibition rate also increased with the increase of irradiation dose. Therefore, these results suggest that GAA can increase the radiosensitivity of HepG2 cells, and HepG2 cells are more sensitive to the high-LET neutron irradiation than low-LET \gamma -ray irradiation. As such, this study suggests that GAA not only may serve as a natural radiosensitizer, but also may provide a new radio-therapy method which may be suitable for cancers, or particularly, liver cancers.