This study determined the effects of added buckwheat extract on the rate of corn starch hydrolysis in vitro as well as blood glucose responses through its supplementation in healthy subjects. The rate of corn starch hydrolysis in the presence or absence of various buckwheat extracts was determined in an in vitro enzyme/dialysis system for 2 hr. The buckwheat was extracted by water, ethanol(40%, 70%, 100%) and methanol(40%, 70%, 100%), respectively. Twenty percent(w/w) additions of the ethanol, methanol and water buckwheat extract to corn starch solution significantly reduced the starch hydrolysis at every minute for 2 hr(p<0.05). The calculated hydrolysis indices of the buckwheat extracts were in the order of 100% ethanol extract(50), 100% methanol(54), 40% ethanol(58), 40% methanol(62), 70% methanol(64), 70% ethanol(68), water (82). For the blood glucose response study, groups of 12 volunteers were given 50 g of boiled rice with or without buckwheat extract(10% and 20% of starch weight) using the 100%, 70%, and 40% ethanol extracts, respectively. The addition of each buckwheat ethanol extract significantly reduced blood glucose concentrations at three or more points during 2 hr and also reduced the mean peak rise and area under the blood glucose curve(p<0.05). The calculated glycemic index(GI) values for all ethanol buckwheat extract groups were significantly decreased compared to the control(rice). At the concentrations of 20%, the buckwheat 100% ethanol extracts lowered the GI by 68%. The 100% ethanol extract was more effective than the 70% and 40% extracts for reducing GI. Therefore, the 100% ethanol buckwheat extract would be the most therapeutically useful in modifying postprandial hyperglycemia.