Antioxidant activity of the ethanol extract from boiled-water of Hizikia fusiformis (EBH) were compared with those of BHA, L-ascorbic acid, gallic acid, caffeic acid and (-)-catechin. The free radical scavenging ability against DPPH (1,1-diphenyl-2-picrylhydrazyl), authentic peroxynitrite and reducing power were measured as indices of antioxidant activity. EBH showed the potent DPPH radical and peroxynitrite scavenging activities, showing 85.23 and 96.97% at final concentration of $1000{mu}g/ml$, respectively. The reducing power increased with the increasing amount of EBH (final concentration of 1, 10, 100 and $1000{mu}g/ml$). Total phenolic content of EBH was 588 mg (-)-catechin/g at the final concentration $1000{mu}g/ml$. Total phenolic contents correlated with DPPH radical scavenging activity $(R^2=0.766)$ and reducing power $(R^2=0.944)$. These results suggested that EBH could be a natural antioxidative source containing antioxidative components.