Objective: To investigate the potential radioprotective effect of tetrahydropalmatine (THP) against Irradiation induced lung injury (RILI). Methods: Sprague-Dawley rats were divided into 4 groups: a control group, an irradiation (R) group, a R + THP group and an only THP group. The rats were irradiated with a single dose of 15 Gy over their right hemithorax. After irradiation at 1, 4, 8, 12, 16 weeks, rats of each groups were killed. Lung tissues were detected by histopathologic changes with HE. Using TUNEL staining and electron microscopy to explore the effectiveness of THP displayed on irradiation induced pulmonary cells apoptosis. Another part of lung tissues were used to detect MDA and SOD level. The level of TGF-β1 in serum was detected with ELISA (P<0.05). Results: The lung of control group and THP alone group showed no inflammatory reaction. Different levels of inflammation were found in the lung tissues of the rats of R group and R + THP group in 4 to 16 weeks after irradiation, the deleterious effects of irradiation significantly ameliorated in THP group. Compared R + THP group with R group at the same period, in lung tissue, the level of MDA increased significantly, the level of SOD increased significantly, the level of TGF-β1 in serum decreased significantly, and inflammation factors was not much. The difference of the level of TGF-β1, the level of MDA and SOD in lung tissue, and the apoptosis index of lung tissue in different times between groups was statistically significant (P<0.05 or P<0.01). Conclusion: THP can markedly attenuate RILI through anti-apoptosis, anti-fibrosis and anti-oxidative injury effect.