This study was conducted to investigate the effect of oligosaccharide on the reduction of oxidative stress. Sprague-Dawley rats were fed an AIN-93G diet or a diet containing 5% soybean oligosaccharide for 6 weeks. Each group was divided into two sub-groups after streptozotocin (STZ) injection and fed the control diet or the diet containing oligosaccharide for the next 12 days. The number of fecal bifidobacteria increased significantly in groups fed oligosaccharide diet. Elevated blood glucose concentration after STZ injection declined faster in the oligosaccharide fed group. Liver thiobarbituric acid reactive substance concentration, as an indicator of oxidative stress, did not increase in groups fed the oligosaccharide diet after the STZ injection. In addition, these groups had significantly higher glutathione peroxidase activity both in the plasma and the liver than groups fed the control diet. The results of this study suggest that soybean oligosaccharide has a beneficial effect in reducing oxidative stress in streptozotocin-injected rats.