Oxidative stress occurs in patients undergoing coronary artery bypass operation. The aim of this study was to investigate the difference in oxidative stress in off-pump versus on-pump coronary artery bypass surgery. In the present study, in serial blood samples, plasma malondialdehyde (MDA) as index of lipid peroxidation, red blood cells glutathione peroxidase (GPx) and superoxide dismutase (SOD) were measured to compare the extent of oxidative stress in 30 patients undergoing OPCAB (off-pump coronary artery bypass grafting), 12 patients undergoing CABG (on-pump coronary artery bypass grafting) and 18 healthy controls. In CABG group, MDA levels increased significantly from $2.87{pm}0.62;nmol/mL$ before anesthesia and $2.87{pm}0.65;nmol/mL$ after anesthesia to $3.05{pm}0.66;nmol/mL$ after ischemia (p < 0.05). Similarly, SOD levels also elevated significantly from $661.58{pm}78.70;U/g$ Hb before anesthesia and $659.42{pm}81.21;U/g$ Hb anesthesia induction to $678.08{pm}75.80;U/g$ Hb after ischemia (p < 0.01, p < 0.01, respectively). In OPCAB group, only SOD levels increased from $581.73{pm}86.24;U/g$ Hb anesthesia induction to $590.90{pm}88.90;U/g$ Hb after reperfusion (p < 0.05). Glutathione peroxidase levels were not changed according to blood collection times in both of CABG group or OPCAB group (p > 0.05). Our results show that only mild signs of oxidative stress is found after reperfusion in OPCAB operation compared with CABG operation. Further studies are needed in order to confirm this hypothesis.