Atrial fibrillation (AF) is a common arrhythmia that occurs postoperatively in cardiac surgery. There is evidence for the role of oxidative stress in the etiology of AF. In our study, we examined whether antioxidant ascorbic acid (vitamin C), could help in the reduction of the incidence of postoperative AF. Patients who were scheduled to undergo elective isolated on-pump coronary artery bypass grafting (CABG) were included in our study. One hundred and seventy patients were randomly divided in two groups: Group A (n=85) received vitamin C preoperatively and postoperatively whereas Group B (n=85) did not receive any (control group). The incidence of AF was 44.7% in the vitamin C group and 61.2% in the control group (P=0.041). The hospitalization time, the intensive care unit stay and the time interval for the conversion of AF into sinus rhythm was significantly shorter in the vitamin C group. Patients that developed AF also had longer hospital length of stay (9.5±2.8 days vs. 6.7±1.9, P=0.034). Supplementation of vitamin C reduces the incidence of postCABG AF, and decreases the time needed for rhythm restoration and length of hospital stay.