Oxygen therapy is clear benefit in the treatment of tissue hypoxia, but high concentrations and long term exposures of oxygen carry the risks of detrimental physiologic changes and tissue damages. Tissue damages involve decreased surfactant production,alveolar edema, and alveolar hemorrhage. In spite of these damage, diagnosis of oxygen toxicity is difficult in many clinical settings. In this study, we measured lung compliance to evaluate the oxygen toxicity in 60 white rat of 100-150 g body weight Rats were divided into 6 groups. Group I was control graup, exposed to room air, group II -group V were exposed to 100% oxygen, group II for 24 hours, group IlI for 48 hous, group IV for 72 hours, and group V for 96 hours. Group VI was exposed to oxygen in serial as 30% oxygen for 24 hours, 60% oxygen for 24 hours and then 100% oxygen for 96 hours. After oxygen exposure we obseved changes of lung compliance and pathologic findings, compliance of group V was 15.94 mL/cmH2O significantly lower than that of eontrol group (p<0.05) and lung weight of group V was 2.16 g/100 g heavier than the other groups (p<0.05). Although we found oxygen tolerance was developed in group V, we could not differentiate the degree of pathologic damage cauesd from oxygen toxicity by pathologic findings. We concluded that the measurement of lung compliance is valuable in diagnosis of oxygen toxicity and thus in reducing lung damages.