The repeated silica gel colum chromatographies of n-BuOH fraction obtained from MeOH extracts of Eucommia ulmoides leaves led to isolation of a flavonoid-glycoside inhibiting monoamine oxidase B activity. Its chemical structure was determined to be 3-Ο-[β-D-glucopyranosyl (1→2) β-D-xylopyranosyl] quercetin through interpretation of spectral data and adaptation of acid hydrolysis. The IC_(50) value of this compound in rat brain mitochondrial MAO-B inhibitory activity was evaluated to be 8.05 μmol/ℓ.