The protective effects of sea tangle (Laminaria japonica) extract and fucoidan components on the attack of oxygen radicals in kidney were studied, Sprague-Dawley (SD) male rats (210 $pm$ 5 g) were with fed experimental diets of Dasi-Ex group (sea tangle extract powder of $4.0\%$ added to control diet), Euco-I, II and III groups (fucoidan powder of 1, 2 and $3\%$, respectively, added to Dasi-Ex group) for 45 days, Hydroxyl radical formations were significantly decreased ($10~15\%$ and $15~30\%$) in mitochondria and microsomes of Dasi-Ex and Fuco-I, II, III groups compared with control group. Hydrogen peroxide formations were also significantly decreased ($10~15\%$) in microsomes of Dasi-Ex and Fuco-I, II, III groups compared with control group. Significant differences in mitochondrial basal oxygen radical (BOR) and microsomal induced oxygen radical (IOR) formations of Dasi-Ex and Fuco-I groups could not be obtained, but mitochondrial BOR and microsomal IOR formations were significantly decreased ($12~15\%$ and $13~14\%$) in Fuco-II and III groups compared with control group. BOR formations were significantly decreased ($12~25\%$) in microsomes of Dasi-Ex and Fuco-I, II, III groups, and IOR formations were also significantly decreased ($10~15\%$) in mitochondria of Fuco-I, II, III groups compared with control group, Significant differences in mitochondrial Mn-SOD activities of Dasi-Ex group could not be obtained, but mitochondrial Mn-SOD activities were dose-dependently increased by $8\%,;16\%$ and $36\%$ in Fuco-I, II and III groups compared with control group, Mn-SOD activities an microsome were significantly increased about $20\%$ in Dasi-Ex group, while they were remarkably increased about $40\%$ in Fuco-I, II and III groups compared with control group. lipid peroxide contents were significantly decreased about $15\%$ and $15~25\%$ in mitochondria and microsomes of Fuco-II and III groups. Membrane fluidities resulted in marked increases ($20~35\%$ and $17~24\%$) in mitochondria and microsomes of Dasi-Ex and Fuco-I, II and III groups. These results suggest that administrations of fucoidan added to sea tangle may play a pivotal role in attenuating attack of oxygen radicals in kidney.