In the present study, the hypothesis that quercetin liposomes are able to effectively protect against radiation-induced pulmonary injury in a murine model was tested. C57BL/6J mice receiving whole-thorax radiotherapy (16 Gy) were randomly divided into three groups: control, radiation therapy plus saline (RT+NS) and RT plus quercetin (RT+QU). At 1, 4, 8 and 24 weeks post-irradiation, lung injury was assessed by measuring oxidative damage and the extent of acute pneumonitis and late fibrosis. In the lung tissues from the RT+NS group, the malondialdehyde (MDA) levels were significantly elevated and superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX) activities were significantly reduced; the total cell counts and inflammatory cell proportions in the bronchoalveolar lavage fluid (BALF), plasma tumor necrosis factor (TNF)-α and transforming growth factor (TGF)-β1 concentrations and the hydroxyproline (HP) content were significantly increased. Quercetin liposome administration significantly reduced the MDA content and increased SOD and GSH-PX activities in the lung tissues, and reduced the total cell counts and inflammatory cell proportions in the BALF, plasma TNF-α and TGF-β1 concentrations and the HP content in the lung tissues. A histological examination revealed suppression of the inflammatory response and reduced TGF-β1 expression and fibrosis scores. Radiation-induced oxidative damage ranged from pneumonitis to lung fibrosis. Quercetin liposomes were shown to protect against radiation-induced acute pneumonitis and late fibrosis, potentially by reducing oxidative damage.