BACKGROUND: The radiation-induced lung injury is a common complication from radiotherapy in lung cancer. CpG ODN is TLR9 activator with potential immune modulatory effects and sensitization of radiotherapy in lung cancer. This study aimed to examine the effect of CpG ODN on acute radiation-induced lung injury in mice. METHODS AND RESULTS: The mouse model of radiation-induced lung injury was established by a single dose of 20 Gy X-rays exposure to the left lung. The results showed that the pneumonia score was lower in RT+CpG group than in RT group on 15th and 30th days. Compared with RT group, CpG ODN reduced the serum concentrations of MDA (P < 0.05) and increased the serum concentrations of SOD, GSH (P < 0.05). The serum concentration of TNF-&#945; in RT+CpG group was lower on 15th and 30th days post-irradiation (P < 0.05). CONCLUSION: The study demonstrated that CpG ODN has preventive effects of acute radiation-induced lung injury in mice. Lung inflammatory reaction and oxidative stress are promoted in the initiation of radiation-induced pneumonia. CpG ODN may reduce the injury of reactive oxygen species and adjust the serum TNF-&#945; concentration in the mice after irradiation, which reduces the generation of the inflammatory cytokines.