A new bio-active flavonol glycoside was isolated from the stems of Butea superba Roxb, 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 compound 1 showed antimicrobial activity against plant pathogenic fungi Trich viride, Asprgillus fumigatus, A. niger, A. terreus, Penicillium expansum, Helmitnhosporium oryzae, Botxitis cinerea, Rhizopus oligosporus, R. chinensis, Kelbsiella pneumoniae, Fusearium moniliforme and grampositive bacteria Streplococcus pyogenes, Staphylococcus aureus, Bacillus subtilis gram-negative bacteria Escherichia coli, Proteus vulgaris, Klebsiella pneumoniae, Pseudomonas aeruginosa. The maximum inhibitory effect was shown by H. oryzae, A. niger, B. cinera and gram-positive bacteria.