Background: Obtaining the comfort in intensive care unit to patients is important. Patient-ventilator dyssynchrony is a big problem in respiratory ICU and when a chest physician facing it in ICU must be aware about causes and deleterious effects on the patients, so study of causes and its impact on weaning outcomes is very important. Aim of the work: was to study patient ventilator dyssynchrony, causes and effect on weaning in mechanically ventilated patients in respiratory ICU. Patients and Methods: Fifty patients were included in the study admitted at Sayed Galal, Al-Azhar University Hospital during period from  May 2014 - December 2017  in RICU, all patients underwent the following ,Full history taking , Genral ,Local examinations, Monitoring  for occurance of  agitation,  patient ventilator  dyssynchrony, and duration of weaning, and observation for the cause of the problem. Exclusion criteria, shock patient with haemodynamic instability. Results:The diagnosis of cases 18  COPD, 9 pneumonia, 8 bronchectasis, 4 Ild (interstitial lung disease), 6 OHS(obesity hypoventilation syndrome)  and 5 pulmonary edema. Patients with dyssncrony have more duration of weaning 5.5455+ .90453 days than patients without dyssnchrony have duration of weaning 3.8125+.83417 days with significant difference.  And the dyssnchrony group have more duration of ICU stay time (days) with  9.8485+1.03444 days  than  the patients without dyssnchrony have duration of weaning 8.0625+.85391 days with significant difference  . Conclusion:Patient ventilator asynchrony occurs frequently, it is sensitive to ventilator settings and artificial airway problem, increased secretions, bronchospasm, alteration in body posture and seems to be associated with prolongation of weaning time.