Background: Conventional ultrafiltration (CUF) and modified ultrafiltration (MUF) are routinely used in pediatric cardiac operations to reduce fluid retention in the body. Further amounts of fluids can usually be retracted through MUF than CUF. The aim of this study was to evaluate the effects of MUF on arterial blood gas in children after cardiopulmonary bypass (CPB). Methods: Forty-two patients that underwent cardiac surgery with CPB were divided into 2 groups of CUF MUF (n=21) and CUF (n=21). Arterial blood gas, chest tube drainage, blood transfusion, and dysrhythmias were assessed before the induction of anesthesia, at the start of CPB, at the end of CPB, at the end of MUF, on intensive care unit admission and 2, 4, 6, 8, 10, 12, 24, and 48 hours after surgery. Results: Our study showed statistically significant differences at the end of MUF between the CUF MUF and CUF groups regarding the levels of hematocrit (P=0.02), PO2 (P<0.01), lactate (P<0.05), hemoglobin, O2 saturation, and blood sugar. There were also significant differences between the groups over the 48 hours in chest tube drainage (P=0.01), blood transfusion (P=0.04), and dysrhythmia (P=0.005). The blood levels of electrolytes (K, Na, Ca, and Cl) and other parameters of arterial blood gas were similar between the 2 groups. Conclusions: The administration of CUF MUF was effectively able to decrease bleeding and reduce transfusion requirement. Additionally, it significantly augmented the parameters of arterial blood gas after surgery. (Iranian Heart Journal 2017; 18(2):6-16)