Background: Patients receiving mechanical ventilation have an increased risk of complications resulting in excess morbidity and mortality. Aim of Study:This study aimed to identify the acute effect of chest physical therapy on arterial blood gases of mechanically ventilated patients.Material and Methods: Sixty mechanically ventilated patients participated in the study. Their age ranged from 40 to 60 years. The patients were assigned into one study group including both sexes who received a chest physiotherapy protocol in form of (manual hyperinflation, vibration, percussion, suctioning, upper and lower limbs exercise and ending position) for one session. Arterial Blood Gases (ABG) including (pH, PaO 2 , SO 2 , PaCO2 and HCO 3 ) were assessed by arterial blood sample analysis through the Laboratory GEM premier 3000 device. All the assessment was done for every patient pre and post chest physiotherapy protocol. Results:The results revealed a marked improvement in arterial blood gas exchange as compared to base line reflected by a highly significant statistical increase in PaO 2 and SaO 2 (p<0.001) and a significant decrease in PaCO 2 (p<0.001) while with no significant changes regarding pH and HCO 3 (p=0.36 and 0.15 respectively). Conclusion:Chest physiotherapy protocol is an effective method for improving arterial blood gases of mechanical ventilated patients acutely.