This study was conducted on thirty mechanically ventilated patients with ARF on top of COPD admitted in RICU patients were divided into 3 groups according to mode of ventilation.Group I: included (CMV) volume controlled.Group II: included (CMV) pressure controlled.Group III: included pressure support ventilation.Prone positioning is one of the salvage methods for improning oxygenation in patients with acute respiratory failure. In the prone position, the weight of the heart is on the sternum, this may cause the pleural pressure to be less positive in the dependent areas and hence decrease atelectasis of alveoli. Thus, there appears to be a more homogenous distribution of regional ventilation in the prone position than in the supine position. Statistically significant differences were detected in pa CO2 in cases of group I and also those of group III in comparing supine to prone positions, results, enfavouring the prone position. Statistically significant difference was detected in C stat in group I in comparing the mean of C stat in sitting and position enfavouring the sitting position.