Background: Capnography has made steady inroads in the ICU and is increasingly used for all patients who are mechanically ventilated. There is growing recognition that capnography is rich in information about lung and circulatory physiology and provides insight into many diseases and treatments. Aim and objectives: to evaluate the Role OF Capnography in Weaning in Mechanically Ventilated COPD Patients. Subjects and methods: Comparative prospective single group observational study was conducted in the Critical Care Department – Benha University Hospitals. The study was conducted upon fifty mechanically ventilated patients (36 males &14 females) Results: there was a statistically significant difference between the studied population regarding correlation coefficient between PaCO2 and PETCO2 before and after weaning, validity of PaCO2 and PetCO2 in prediction of successful weaning and comparison between patients with successful and failed weaning regarding outcome except in SOFA and APACHE score. There was no statistically significant difference between the studied population regarding comparison between patients with successful and failed weaning regarding vital signs and comparison between patients with successful and failed weaning regarding blood gases except in PaCO2 and HCO2. Conclusion: the present study found that PacO2 and PetCO2 are correlated to each other before, during and after SBT. Most of the studies that was found reported that PetCO2 is highly correlated with PaCO2 and that PetCO2 may be a rapid and reliable predictor as arterial PaCO2 in respiratory distress.