Objective · To observe the similarities and differences in neonatal rats and immature rats with hyperoxia-induced lung injury and to preliminary explore the role of Rho/Rock signaling pathway in age-related hyperoxia-induced lung injury. Methods · Sprague-Dawley neonatal rats and immature rats aged 3 weeks were randomly assigned to the neonatal air group, the neonatal hyperoxia group, the immature air group, and the immature hyperoxia group. The changes in survival rate and body mass were statistically observed form the construction of animal model till 14 d. Lung tissues of SD rats were harvested. Pathological examination and injury scoring were performed and hydroxyproline, SOD, MDA, ROCK1, p-MYPT1, and MYPT1 were examined. Results · ① The neonatal hyperoxia group had a significantly lower survival rate than the neonatal air group, the immature air group, and the immature hyperoxia group. There was no significant difference in survival rate between the immature air group and the immature hyperoxia group. The body weight increased slower in the neonatal and immature hyperoxia groups than in the neonatal and immature air groups. The body weight increased slower in the neonatal hyperoxia group than in the immature air group. ② The neonatal hyperoxia group and immature hyperoxia group developed lung injury and fibrosis. The neonatal hyperoxia group was more serious than the immature hyperoxia group. ③ The neonatal and immature hyperoxia groups had higher MDA level and lower SOD activity than the neonatal and immature air groups. The neonatal hyperoxia group had lower SOD activity than the immature hyperoxia group. The SOD activity was significantly decreased after hyperoxia treatment. ④ The neonatal and immature hyperoxia groups had higher ROCK1 and p-MYPT1 expressions than the neonatal and immature air groups. The neonatal hyperoxia group had higher p-MYPT1 expression than the immature hyperoxia group. Conclusion · Hyperoxia-induced lung injury is age-dependent. Neonatal rats have significantly weaker hyperoxia tolerance than immature rats and this is associated with the weak antioxidant capacity of their own. The degree of activation of Rho/Rock signaling pathway may play an important role in age-related hyperoxia-induced lung injury.