An inhibitory activity against ${alpha}$-glucosidase was identified in extracts of medicinal lichens, Parmelia austrosinensis and P. praesorediosa. The extracts retained almost all of their original activities when treated with heat, acid and alkaline conditions, and some hydrolytic enzymes. Partially purified inhibitor showed strong inhibition against disaccharide hydrolytic enzymes of mammalian and mold origin, but weak or no inhibition against polysaccharide hydrolytic enzymes except glucoamylase. The inhibitors from the two Parmelia sp. showed almost same retention time in HPLC. The inhibitor suppressed elevation of blood glucose level in rats after oral administration of soluble starch or sucrose.