This study investigated the hypoglycemic effect of fermented soymilk extracts added with bokbunja (FSB) in streptozotocin (STZ)-induced diabetic mice. FSB was prepared via fermentation with Bacillus subtilis of soymilk added with bokbunja extract and its extraction with methanol. The FSB showed higher (p<0.05) inhibitory activities than fermented soymilk extracts (FS) against $alpha$-glucosidase. The $IC_{50}$ values of 5% FSB for $alpha$-glucosidase was 0.55 mg/mL, which was superior to that of FS (0.77 mg/mL). The increase of postprandial blood glucose levels were more significantly suppressed in the 5% FSB-administered group than control or FS group in the streptozotocin induced diabetic mice. Moreover, the area under curve (AUC) was significantly reduced via 5% FSB administration in the diabetic mice as well as it delayed absorption of dietary carbohydrates. Results from this study suggest that FSB may help decrease the postprandial blood glucose level via inhibiting $alpha$-glucosidase.