Cellulose was extracted from Saccharina japonica(kelp) and Undariapinnatifida(wakame), two major edible brown algae in China, and the structural features were analyzed and compared. The kelp had significantly higher cellulose content than that of the wakame, and the cellulose extraction yieldswere 5.59% and 2.61% of the dry seaweeds, respectively. The purity of the cellulose samples reached 99.86% and 98.91% respectively according to acid hydrolysis-HPLC analysis and FT-IR analysis. The kelp cellulose had a degree of polymerization of 813, significantly higher than that of the wakame of 315. X-ray powder diffraction analysis revealed natively highly crystalline structure of the seaweed cellulose, with the crystallinity indexes reaching 72.06% and 71.48%, respectively, which were comparable to that of the commercial microcrystalline cellulose derived from higher plant. Further analysisof theX-raydiffractograms suggested that the crystalline region of the seaweed cellulose featuredIα (one-chain triclinic) phase-rich structure, evidently different from Iβ (two-chain monoclinic) phase-rich structure of cellulose from higher plant.