In this paper, the estimation of central venous pressure by measuring the IVC Collapsibility Index has been investigated by Sonography. In this study, the patient was submerged and intravenous pressure was determined by central venous catheter in inhale and exhale mode (CVP). At the same time, the lower anterior posterior vein diaphragm was recorded by Sonography. Inhale and exhale measurements are performed and the collapsibility index is calculated (divergence of the IVC diameter at the inhale and exhale divided by the exhale mode IVC diameter). Finally, the variables were analyzed using statistical software SPSS 22 and statistical analysis. The results of the study showed that the most common cause of patients admitted to the intensive care unit was cancer patients, gastrointestinal bleeding and pneumonia, respectively, with a prevalence of 16%, 12% and 12%. The mean central venous pressure in 50 patients was 10.7 cm in water and the minimum and maximum central venous pressure and standard deviation were 2 and 29 cm water and 6.14%, respectively. In the under study patients, the mean, minimum and maximum diameter of lower vein in the inhale by Sonography were 5.9, 12.4 and 25.5 respectively, with a standard deviation of 3.81. The results of this study showed a direct and significant correlation between IVC collapsibility index and central venous pressure (P-value = 0.009 and r = -0.367). As a result, measuring the lower vein diameter by sonography was used as a non-invasive and reliable method for estimating the volume of intravascular fluid to evaluate the response to treatment in patients admitted to the intensive care unit rather than invasive methods.