The ethanol (EtOH) extract of the branches of Cotinus coggygria (Anacardiaceae) exhibited a significant inhibition on the yeast $alpha$-glucosidase, one of the key enzymes related with diabetes mellitus, in a dose dependent manner, in vitro. The intensive phytochemical survey of the EtOH extract of the species by way of bioactivity-guided fractionation resulted in the isolation of 1,2,3,4,6-penta-O-galloyl-$eta$-D-glucose (1) as an active principle responsible for the inhibition on $alpha$-glucosidase, together with two related components 2 and 3. Compound 1 demonstrated a strong inhibition on the yeast $alpha$-glucosidase, in vitro and $IC_{50}$ value was calculated as 0.96 mg/ml, when that of a reference drug, acarbose was estimated as 5.3 mg/ml. On the other hand, other related constituents of the species, 1,2,3,6-tetra-O-galloyl-$eta$-D-glucose (2) and gallic acid (3) were exhibited relatively poor inhibition upon the yeast $alpha$-glucosidase, respectively.