Silica gel chromatography HPLC, El-Mass, and NMR analyses were performed in order to determine the structure obtained from the separation and refinement of the main components of an ethanol extraction of Sargassum siliquastrum, which indicated its antioxidant activity An ethanol extraction of Sargassum siliquastrum demonstrated the strongest antioxidant activities of ethyl acetate when isolated by silica gel chromatography This amounts to 84.9 +/- 1.2% of its 0.5 mg ml(-1) concentration. The thiobarbituric acid reactive substances (TBARS) measurements from 3 isolations of an HPLC separation of ethyl acetate showed that the antioxidant activity of peak 2 was the most dominant at 84.08% in a 0.5 mg ml(-1) concentration. Peak 2 was verified as a type of chromene through El-Mass and NMR analysis. Its metal sealing characteristic was low while its characteristic of TBARS and DPPH erasure indicated similar or higher levels of metal sealing characteristic. The NMR spectroscopic data was used to elucidate the structure of this new compound, which showed strong antioxidant activity in the assay.