Background: In clinical area, assessment of body fluid and determination of the intravascular volume after major surgeries such as heart surgery is a significant challenge. The initial purpose of intravascular volume assessment in patients with hemodynamic instability is to determine whether they would benefit from fluid administration or not. Objectives: This study aims to compare pulse pressure variation (PPV) compared to thoracic fluid content in checking out and optimizing fluid volume in mechanically ventilated patients admitted to intensive care unit after cardiac surgery. Methods: In the present study a prospective descriptive-analytic design was used. Thirty mechanically ventilated patients admitted to intensive care units of Rajaie heart center, Tehran, Iran, were recruited in the current study after cardiac surgery based on inclusion criteria. Data collection tools included demographic and clinical data sheets. Hemodynamic parameters such as CVP and pulse pressure were recorded by bedside monitoring. Thoracic fluid content (TFC) was measured by non-invasive continuous cardiac output monitoring (NICCOMO) system. Data were analyzed in SPSS version 15, using statistical tests.