A method is described to comprehensively evaluate the oxidative stress caused by postprandial hyperglycemia. The hemolysis of erythrocytes collected from the fingertip in the glucose tolerance test was used as an index of oxidative stress in the subjects, and the method was used to examine the effect of ingesting barley. Both the blood glucose level and the area under the curve (AUC) ratio of the hemolysis rate measured after ingesting rice tended to be proportional to an increasing amount of rice. When those subjects whose AUC ratio had increased after ingesting rice (360 g) took a mixture of rice and barley, no significant increase in the AUC ratio was apparent, while the blood glucose level was similar when compared to only ingesting rice. Although the bioactive components of barley rice responsible for the antioxidative effect need to be identified, these results indicate that the barley intake could have had a beneficial effect on the endothelial dysfunction caused by oxidative stress.