This study was designed to investigate whether the brown alga gamtae (Ecklonia cava) may inhibit ${alpha}$-glucosidase and ${alpha}$-amylase activities, and alleviate postprandial hyperglycemia in streptozotocin-induced diabetic mice. For that purpose, we prepared an enzymatic hydrolysate from gamtae (EHG) by using the carbohydrase, Celluclast. EHG evidenced prominent inhibitory effect against ${alpha}$-glucosidase and ${alpha}$-amylase. The $IC_{50}$ values of EHG against ${alpha}$-glucosidase and ${alpha}$-amylase were 0.62 and 0.59 mg/mL, respectively, which evidenced the higher activities than that of acarbose. EHG did not exert any cytotoxic effect in human umbilical vein endothelial cells (HUVECs) at various concentrations (from 0.25 to 2 mg/mL). The increase of postprandial blood glucose levels were significantly suppressed in the EHG administered group than those in the streptozotocin-induced diabetic or normal mice. Moreover, the area under curve (AUC) was significantly reduced via EHG administration (6,102 vs. 10,425 $mg{cdot}min$/dL) in the diabetic mice as well as it delays absorption of dietary carbohydrates. These result indicated that EHG might be a potent inhibitor for ${alpha}$-glucosidase and ${alpha}$-amylase.