Aim: Radiation-induced lung injury (RILI) is a major dose-limiting factor during thoracic irradiation. Imiquimod (Imq) is a heterocyclic amine that has been shown to be effective in diseases related to inflammation and fibrosis. In this study, we aimed to evaluate the impact of Imiquimod on RILI. Material and Method: The study included 60 adult female Wistar-Albino rats (250-300 g). Rats were divided into 6 groups: Group (G) 1: control, G2: radiotherapy (RT) only, G3 and G4: 5 and 10 mg/kg Imq; G5 and G6 RT: plus 5 and 10 mg/kg Imq groups respectively. A single dose of 15 Gray (Gy) RT was given to the lungs. Imq was applied intraperitoneal. Results: The inflammation, fibrosis, and transforming growth factor (TGF)-beta scores of the study groups were significantly different at 6th and 16th week of RT (p < 0.001 for all). At the 6th week of RT, inflammation, fibrosis, and TGF-beta scores did not differ in both RT and non-RT groups. By the 16th week of RT inflammation, fibrosis, and TGF-beta scores were significantly different between G2 and G5, and G2 and G6. Electron microscopy findings supported the results of the light microscopy. Discussion: Although Imq did not improve pneumonitis phase, Imq attenuated radiation-induced lung fibrosis. These findings should be clarified with further preclinical and clinical studies.