Boiled-water extract (BWE) of Hizikia fusiformis has antioxidant effects. To investigate antioxidant effects of BWE, 1,1-dephenyl-2-picrylhydrazyl (DPPH) and thiobarbituric acid (TBA) methods were used. BWE exhibited strong radical-scavenging activity (87.3% at final conc. $2000;{mu}g$ BWE/ml in an assay mixture) on DPPH and good inhibitory activity (64% at final conc. $1000;{mu}g$ BWE/ml in an assay mixture) on lipid peroxidation. Raw 264.7 cells treated with lipopolysaccharide (LPS) showed 26.7-fold increase in nitric oxide (NO) ($48.9;{mu}M$) compared with control group ($1.83;{mu}M$). When treated with BWE and LPS, NO production was inhibited by BWE dose-dependently.