One lung ventilation(OLV) during thoracotomy is frequently used for the purpose of facilitating surgical exposure by collapsing the lung in the operative hemithorax. But severe hypoxemia may occur during OLV inspite of higher inspired oxygen concentration. This study was performed to evaluate the effect of positive end-expiratory pressure(PEEP) level to the ventilated lung on the arterial oxygenation in the thoracotomy patients(n-10) who showed PaO2 below 100 mmHg during one lung ventilation (OLV) at the F1O2 = 1.0 . After measuring control value of arterial blood gas(ABGA), peak inspiratory pressure and hemodynamic parameter (mean arterial pressure and heart rate), PEEP device 5 cmH2O and then 10 cmH2O was applied to the expiratory breathing circuit for 10 min at each pressure setting. Data of above parameter was collected after 10 min each PEEP application. There were no siginificant changes in the mean arterial pressure and heart rate between control, PEEP 5 cmH2O and PEEP 10 cmH2O. Although PaO2 did not significantly increased with PEEP 5 cmH2O compared to control value, the application of PEEP 5cmH2O increased PaO2 in 6 patients and decreased in 4 patients. In the PEEP 10 cmH application, PaO2 was significantly improved compared to control and PEEP 5 cmH2O values (78.4±11.6 mmHg, 84.6±19.2 mmHg vs. 95.3±18.5 mmHg). It is concluded that it may be necessary to adjust PEEP level to the ventilated lung to improve oxygenation when hypoxemia occurs during OLV.