Inhibition of xanthine oxidase by seaweed extracts obtained from Undaria pinnatifida, Ecklonia stolonifera, Ecklonia cava, Laminaria japonica, Sargassum, Codiumfragile, Enteromorpha compressa and Porphyra tenera were investigated. Extracts of E. stolonifera and E. mua remarkably inhibited xanthine oxidase activity compared to those of other seaweed. The xanthine oxidase inhibitory activity of E. cava was higher than that of E. stolonifera. Diethyl ether extract from E. cava was more effective in the inhibition of xanthine oxidase than other solvent extracts. Two xanthine oxidase inhibitors(A-1 and A-2) from diethyl ether extract were isolated and purified by silica gel column chromatography, thin layer chromatography and high performance liquid chromatography. Xanthine oxidase inhibitory activities of these compounds were 27.8 and 48.1% per 0.4mg, respectively. The active compound A-2 had absorption peak at 420nm, 456nm and 467nm, which can be considered as siphonaxanthine.