In order to explore the radio-protective mechanism of treatment with nitroxides R-1 on the human liver cell L-02 was used as a model of target, and L-02 cells were divided into the control, drug, irradiation and combination groups. The samples of drug and irradiation groups were given 0.25 μmol / L of R-1 and exposed to 4 Gy of ,60,Co γ-rays irradiation respectively; the combination group received both treatments; whereas there was no any treatment with the control group. The cell cycle was measured by flow cytometry; the enzyme activity of SOD and GSH was detected by chemiluminescence and spectrophotometric method, respectively meanwhile the concentration of MDA and ROS was measured by TBA and DCFH-DA, respectively. The expression of p53, Bcl-2, Bax and caspase-3 protein was observed by western blot. The results show that compared with irradiation group, the G,2, cell cycle arrest was relieved (determined at 48 h, G,2, phase proportion of the cell was (29.4±3.40):(44.01±2.71); ,t,=6.46,p,<,0.05, the enzyme activity of SOD was (97.03±5.99): (50.94±5.47); ,t,=9.84,p,<,0.05 and GSH was (16.56±0.91): (7.49±0.54); ,t,=14.81,p,<,0.05 meanwhile the concentration of MDA was (2.87±0.33): (4.93±0.17); ,t,=9.56,p,<,0.05 and ROS was (45.33±3.79): (75.67±4.7); ,t,=8.67,p,<,0.05. It was found that the expression of p53, Bax and caspase-3 decreased significantly, while the expression of Bcl-2 increased in the combination group. This experiment indicates that nitroxides R-1 can protect the L-02 cell from radiation damage effectively by removing the free radical in cells.