Cellulose was extracted from kelp and wakame, which were two major edible brown seaweeds 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 yields were 5.59% and 2.61% of the dry seaweeds. The purity of the cellulose samples reached 99.86% and 98.91% according to the acid hydrolysis-HPLC analysis and FT-IR analysis. The kelp cellulose had a degree of polymerization of 813, which was significantly higher than that of the wakame of 315. X-ray powder diffraction analysis revealed that two kinds of brown seaweed cellulose both had natively highly crystalline structure, with the crystallinity indexes reaching 72.06% and 71.48%, which were comparable to that of the commercial microcrystalline cellulose derived from advanced plant. Further analysis of the X-ray diffractograms suggested that the crystalline region of the brown seaweed cellulose featured Iα (one-chain triclinic) phase-rich structure, evidently different from Iβ (two-chain monoclinic) phase-rich structure of cellulose from advanced plant. This study provides a reference for resource development of high quality dietary fiber.