The aim of this study is to evaluate the effect of dose rate (DR) on lung tissue. The rats included in the study were randomly grouped into 3 groups: Group (G) 1 was defined as control group, and in this group rats were sham irradiated. G2 was the group receiving a single dose of 12 Gy in DR of 300 monitor unit (MU)/min. G3 was the group receiving a single dose of 12 Gy in DR of 600 MU/min. Radiotherapy (RT) was applied under general anesthesia with 6-MV photon beams to both lungs. At the 6th and 16th week of the RT, animals from each group were killed for light and electron microscopy evaluation. We evaluated the scores of each group in the 6th and the 16th week and found that in G2, there were significant increases in the perivascular fibrosis (P = .018), interstitial fibrosis (P = .002), total inflammation (P = .040), and total fibrosis (P = .003) scores. In G3, we found statistically significant increases in perivascular fibrosis (P = .001), interstitial fibrosis (P = .002), and total fibrosis scores (P = .029). There was no significant difference in the total inflammation score in G3 (P = .225). When we compare G2 and G3 in the 6th week, we found significant increase in the interstitial thickening (P = .039) and total inflammation (P = .035) scores in G3. Dose rate per fraction may have an impact on normal tissue toxicity. The prominent effect of increased DR in lung tissue is fibrosis which should be kept in mind, especially in cases where higher doses per fraction are used.