In order to estimate the effects of the A-ring hydroxyl group of baicalein (5,6,7-trihydroxyflavone, 1) on rat intestinal alpha-glucosidase inhibition, flavone, monohydroxyflavones, dihydroxyflavones, and methylated derivatives of 5,6,7-trihydroxyflavone were used for the structure-activity relationship (SAR) study. The importance of the 6-hydroxyl group of baicalein was validated for an exertion of the activity. And also, the tested flavones which lacked a hydroxyl substituent on any of positions 5, 6, or 7, showed no activity. Hence, the 5,6,7-trihydroxyflavone structure was concluded to be crucial for the potent inhibitory activity. In addition, an introduction of electron-withdrawing or electron-donating groups at position 8 of baicalein led to a dramatic decrease for activity, except for 8-fluoro-5,6,7-trihydroxyflavone, which carried a less bulky substituent on position 8. Hence, this result suggested that a sterically bulky substituent on C-8 of baicalein was detrimental for the activity regardless of its electronic nature. Through examining the inhibitory mechanism of baicalein against rat intestinal alpha-glucosidase, it was suggested to be a mixed type inhibition.