Background: Radiation therapy plays an important role in lung carcinoma treatment. However, the incidence of symptomatic radiation-induced lung injury is high. This study aimed to evaluate radioprotective effects of flavonoids extracted from Astragalus complanatus and mechanisms of action against radiation damage. Methods: Alteration in antioxidant status and levles of several cytokines were investigated in BABL/C mice treated with 4 mg/kg b.wt. flavonoids after exposure to 10Gy thoracic radiation. Results: Serum levels of SOD in the flavonoids+radiation group were significantly higher compared to the radiation control group, while TGF-${eta}1$ and IL-6 were lower. Mice in the radiation control group displayed more severe lung damage compared with the flavonoids+radiation group. The expression of TGF-${eta}1$ and TNF-${alpha}$ in the radiation control group was markedly increased in alveolar epithelial cells and macrophages of the alveolar septum. Conclusions: From the results of the present study, flavonoids could be excellent candidates as protective agents against radiation-induced lung injury.