Abstract： Objective To investigate the role of nitric oxide synthase (NOS) in the pathogenesis of acute lung injury induced by hyperoxia. Methods Seventy - two mice were exposed to oxygen (for 24 to 72 h) in sealed cages ＞95%, the severity of lung injury was evaluated, the expression of iNOS and eNOS in lung tissue at 24,48 and 72 hours of hyperoxia were studied by immunohistochemistry (IHC). Results Acute lung injury was caused hyperoxia,accompanied by increased total cell, macrophage and neutrophil counts in BALF. IHG study showed iNOS and eNOS protein were mainly localized in the cytoplasm of airway epithelial cells, alveolar macrophages and vascular smooth muscle cells. The expression of iNOS and eNOS protein in airway epithelium increased greatly in hyperoxia- induced lung injury. Conclusion NOS played an important role in the development of hyperoxia - induced lung injury in mice lung.