Abstract: By bioactivity-directed fractionation, three antioxidant constituents have been characterized from Sargassum micracanthum collected in Jeju Island, Korea. The potent antioxidant elucidated sargaquinoic acid, sargachromenol and fucosterol by NMR and mass spectroscopic data. The free radical scavenging activities of the sargaquinoic acid, sargachromenol and fucosterol were investigated in relation to 1,1-diphenyl-2-picrylhydrazyl (DPPH) and hydroxyl radicals using an Electron Spin Resonance (ESR) system. The sargaquinoic acid and sargachromenol were found to scavenge DPPH (IC50, 49.3 and 100.2 μM). Therefore, these results resent sargaquinoic acid and sargachromenol with a potent antioxidative activity that could be useful in cosmetics, foods and pharmaceuticals.