Abstract: Objective: To investigate the effect of the rats intermittently exposed to the high oxygen concentration for a period of time on the lungs. Methods: Rats were randomly divided into tow groups, the first group was control group （rats exposed to room air for 7 days）. The second group was Hyperoxic Preconditioning（HOP） group（rats exposed to 80-85% oxygen, 1atm, six hours each day for 7 days before the experiment）.Results: There was no significantly difference between the HOP group and control group in the wet weights of the rat lungs. The antioxidant enzymes in HOP group were elevated significantly. The electron microscopy examination found that there were no damages in the capillary epithelial cells and alveolar type Ι cells. Conclusion: Intermittent high oxygen concentration could elevate the antioxidant enzymes and had no significantly damages in the rat lungs.