A new bio-active flavonol glycoside was isolated from the stems ofButea superbaRoxb, and its structure was determined by spectral analysis and chemical degradations as 3,5,7,3′,4′-pentahydroxy-8-methoxy-flavonol-3-O-β-D-xylopyranosyl(1 → 2)-α-L-rhamnopyranoside. The compound1showed antimicrobial activity against plant pathogenic fungiTrich viride, Asprgillus fumigatus, A. niger, A. terreus, Penicillium expansum, Helmitnhosporium oryzae, Botxitis cinerea, Rhizopus oligosporus, R. chinensis, Kelbsiella pneumoniae, Fusearium moniliformeand grampositive bacteriaStreplococcus pyogenes, Staphylococcus aureus, Bacillus subtilisgram-negative bacteriaEscherichia coli, Proteus vulgaris, Klebsiella pneumoniae, Pseudomonas aeruginosa. The maximum inhibitory effect was shown byH. oryzae, A. niger, B. cineraand gram-positive bacteria.