Barley and wheat were investigated as natural ${alpha}$-glucosidase inhibitors having potent inhibitory activities against the antidiabetic enzymes ${alpha}$-glucosidase, maltase, and sucrase. After soaking with tea catechin as an elicitor, extracts of barley and wheat were prepared using thermal treatment in an autoclave at $121^{circ}C$ for 15 min, then the in vitro antioxidant and antidiabetic activities were determined. The total soluble phenolics contents of barley and wheat during soaking were increased by addition of tea catechin, which also contributed to the peroxyl radical-scavenging activity and the reducing capacity. The rat ${alpha}$-glucosidase, maltase, and sucrase inhibitory activities of barley were increased with an increase in the added tea catechin concentration from 0.1% to 0.5%. Tea catechin may have a role as an elicitor for production of phenolics that exhibit antioxidant activities and rat-intestinal maltase and sucrase inhibitory activities during the soaking process of barley and wheat.