Background: Blood pressure monitoring is essential for hemodynamically ill patients in Intensive Care Unit. Invasive measurement from an arterial line is considered as the method of choice in spite of the errors that may occur due to calibration error, movement artifacts and over or under damping. Furthermore, because of the frequent need for continuous and prolonged monitoring of blood pressure for critically ill patient in cardiac intensive care unit, automated noninvasive blood pressure measurements are commonly used. Objective: To compare Invasive with Non Invasive Blood Pressure. And to observe whether femoral and radial arterial lines can be used interchangeably in cardiac intensive care unit. Methods: 70 patients were divided into two equal groups. First group underwent standard femoral artery catheterization for invasive blood pressure monitoring. The second group underwent standard radial artery catheterization for invasive blood pressure monitoring. A cuff was placed in the arm for noninvasive monitoring for all 70 patients. For each patient data were obtained by performing noninvasive blood pressure measurement for 4 times which was at time of admission to intensive care unit (T1), one hour later (T2), two hours later (T3) and three hours after admission (T4). At the same time the invasive blood pressure measurement was recorded for comparison. Results: There were significant differences between diastolic invasive blood pressure measurement obtained from femoral arterial line or radial arterial line with diastolic noninvasive blood pressure measurement. No significant statistical difference in mean arterial pressure between invasive and noninvasive blood pressure was detected. Conclusion: Noninvasive blood pressure measurement alone is insufficient for monitoring of postoperative patients admitted to cardiac intensive care unit after cardiac operation. And femoral or radial arterial line can be used interchangeably for blood pressure monitoring in cardiac intensive care unit.