Background: Pneumonia is an inflammatory condition of the lung affecting primarily the alveoli, typically symptoms include combination of cough, chest pain, fever, and trouble breathing, Fluid resuscitation in critically ill patients is common and serious challenge so measurements of central venous pressure (CVP), pulmonary arterial catheterization, esophageal Doppler ultrasonography (Ultrasound)and trans esophageal echocardiography may be used to determine the volume status of critically ill patients. Objective: To determine the value of measurement of IVC diameter by ultrasound as a noninvasive tool in assessment of intravascular volume and hydration status in ICU patients with pneumonia. Patients and Methods: This prospective cross-sectional study was carried out in the Intensive Care Units of Chest Department, at Al- Hussein and Bab Al-sha'reia University Hospitals, during the period from November 2019 to September 2020, and included sixty persons: Thirty patients with pneumonia (diagnosed on clinical and radiological basis),  and another thirty apparently healthy subjects collected from Hospital persons or patients relative matching with age, sex and body mass index. Body mass index ultrasound measurement of Inferior vena cava diameter was done for all. Results: There was significance decrease of inspiratory IVC-D in patient's group than control's group (mean inspiratory IVC-D of patients 0.63 cm versus 0.92 cm in control's group). Also, there was significant decrease of expiratory IVC-D in patient's group than control's group (mean expiratory IVC-D in patient's group 1.46 cm versus 1.68 cm in the control's group). The IVC-CI of the patient's group in this study had highly significant increase than control's group (66% versus 46%). Conclusion: Assessment of the physiologic characteristics of the IVC by ultra sound provides a rapid distinction between low and high volume states and offers a rapid, easily, reliable and noninvasive way to guide resuscitation in patients with pneumonia.