SummaryThe inhibitory activity of six groups of flavonoids against yeast and rat small intestinal ␣ -glucosidases and porcine pancreatic ␣ -amylase was compared, and chemical structures of flavonoids responsible for the inhibitory activity were evaluated. Yeast ␣ -glucosidase was potently inhibited by the anthocyanidin, isoflavone and flavonol groups with the IC 50 values less than 15 M . The following structures enhanced the inhibitory activity: the unsaturated C ring, 3-OH, 4-CO, the linkage of the B ring at the 3 position, and the hydroxyl substitution on the B ring. Rat small intestinal ␣ -glucosidase was weakly inhibited by many flavonoids, and slightly by the anthocyanidin and isoflavone groups. 3-OH and the hydroxyl substitution on the B ring increased the inhibitory activity. In porcine pancreatic ␣ -amylase, luteolin, myricetin and quercetin were potent inhibitors with the IC 50 values less than 500 M . The 2,3-double bond, 5-OH, the linkage of the B ring at the 3 position, and the hydroxyl substitution on the B ring enhanced the inhibitory activity, while 3-OH reduced it.