Background: The efficiency of physiotherapy maneuvers for   respiratory   disorders has not been proven because it depends on the proper performance of therapists and the patients' collaboration. Bag squeezing which is done with manual breathing unit (MBU) is a technique that is often studied. Aim of the study: to assess the effect of breath stacking technique on respiratory efficiency, airway clearance and cough intensity in mechanically ventilated patients. Materials and Method: Research design: A research design based on a quasi-experimental approach. Settings: This study was carried out at 2 ICUs at Alexandria Main University hospital. Subjects: A convenience sample of 80 critically ill patients was selected. Tools of the study: two tools were used in this study. Tool I: Patient respiratory efficiency assessment: it was used to assess patients' respiratory efficiency; it consisted of four parts. Part I: Patient characteristics and clinical data.  Part II: Respiratory clinical parameters assessment. Part III: physiological parameters. Part IV: assessment of airway clearance. Tool II: Cough reflex intensity scale: to assess cough strength. Methods: Two groups (group A and group B) were used to divide the subjects of the study. The patients in group A were subjected to breath stacking technique and chest physiotherapy (CPT), while those in group B were received CPT. The researcher assessed patient respiratory efficiency and cough reflex intensity before, immediately after and after 60 mins of performing interventions. Results: it was found that the number of patients in the BS group who had no cough increased from more than half to seven patients after intervention. Simultaneously, the number of patients who had weak audible cough increased from 11 to 20, those with clear audible cough increased from 1 to 8, and others with strong cough increased to 5 with significant difference between groups in relation to cough strength. Conclusion: Stacked breathing technique is notably effective in enhancing cough intensity and airway clearance, thereby promoting respiratory efficiency.