The purpose of this study was to compare the effect of proneposition on the lung mechanics and blood gases exchange inmechanically ventilated chronic obstructive pulmonary disease(COPD) patients relative to the supine position.METHODS: Thirty mechanically ventilated patients with acuterespiratory failure type П on top of chronic obstructivepulmonary disease on a volume controlled mode were put inprone position for one hour then turned back to supineposition . Arterial samples for blood gases analysis "ABG"and Lung mechanics parameters were taken before turning, 30and60 minutes after turning the patient to prone position and 30minutes after returning the patient to supine position.RESULTS : After one hour of pronation the PIP increased from33.23±6.8 to 36.4±6.5, the Pmean increased from 10.1±2.3 to11.74±2.8, Cdyn decreased from22.47±3.9 to 20.77±3.2, thePplate decreased from 24.08±4.1 to 21.8±3.9, the Cstatincreased from 33.36±5.7 to 42.04±8.9, the PaO2 increased from94.83±38.8 to 105±39.4 and the PaCO2 decreased from55.1±12.02 to 50.6±11.08.conclusion: Prone position showed significant decrease inplateau pressure and increase in static compliance, it alsoimproves oxygenation and CO2 wash relative to supine position.The peak inspiratory pressure, mean airway pressure, andintrinsic positive end expiratory pressure increase in proneposition. The effect of prone position on oxygenation and CO2wash lasts for at least on hour after turning the patients tothe supine position.