Introduction:-Chest ultrasound can be performed by critical care physicians at bedside routinely for diagnosis pleural effusion, pneumothorax, lung consolidation and alveolar-interstitial pathology as an alternative to chest x-ray using CT chest as golden standard. Objective: to compare sensitivity and specificity of chest ultrasound and routine daily chest X-ray in diagnosis of chest diseases in mechanically ventilated patients admitted at Critical care department, Benha university hospitals. Patients and methods: the study was conducted upon 50 consecutive mechanically ventilated patients (22 males &28 females) presented to Critical care department, Benha university hospitals with a mean age of 58 years (SD±15.55; (19-82) during the period from July 2016 to April 2017. Chest ultrasound (CUS) examination was done for patients on mechanical ventilation on first day of admission prior to their chest x-ray, and then follow up daily Chest ultrasound (CUS) examination was done over the period of mechanical ventilation with comparison to chest x-ray. Results: CUS was more sensitive in detection of chest diseases. thoracic US was introduced as a dependable diagnostic tool for parenchymal diseases and we have found that the sensitivity washigher with US than CXR (86.8% vs 65.8) while specificity was (100.0 vs58.3 )respectively Through using thoracic US in the diagnosis of pleural diseases,we found that it had a sensitivity of 100.0% and a specificity of 94.7%in comparison to 75.0% sensitivity and92.1% specificity of CXR. Conclusion: The results and advantages of thoracic US make it a suitable diagnostic modality for evaluating lung and pleural pathologies in the ICU that will have the upper hand over CXR .