To control blood glucose level as close to normal is the major goal of treatment of diabetes mellitus. $alpha$-glucosidase is the enzyme to digest dietary carbohydrate and inhibition of $alpha$-glucosidase could suppress postprandial hyperglycemia. The methanol extract of soybean sprout was tested for the inhibitory activities against $alpha$-glucosidase in vitro. Soybean sprout extract inhibited yeast $alpha$-glucosidase activity by 24.5% at the concentration of 5 mg/mL. The methanol extract of soybean sprout was subsequently subjected to sequential fractionation with hexane, ethyl acetate, butanol and water. Among the fractions tested ethyl acetate-soluble fraction showed relatively strong inhibition against $alpha$-glucosidase by 36.3% at the concentration of 5 mg/mL. Acarbose, standard $alpha$-glucosidase inhibitor, inhibited $alpha$-glucosidase activity by 40.1%. The ability of soybean sprout extract to lower postprandial glucose was studied in streptozotocin-induced diabetic rats. Starch solution (1 g/kg) with and without the methanol extract of soybean sprout (500 mg/kg) was administered to diabetic rats after an overnight-fast by gastric intubation. A single oral dose of soybean sprout extract inhibited the increase in blood glucose levels significantly at 60, 90, 120, 180 min (p<0.05) and decreased incremental response areas under the glycemic response curve significantly (p<0.05). These results suggest that soybean sprout might exert hypoglycemic effect by inhibiting $alpha$-glucosidase activity.