Five Fucoidan fractions from Laminaria japonica with different sulfate content and molecular weight were obtained by anion-exchange chromatography and mild acid hydrolysis. Their antioxidant effects on azo radicals AAPH- and AMVN -induced oxidation of human low-density lipoprotein (LDL) were evaluated by monitoring cholesteryl ester hydroperoxides (CHE-OOH) formation kinetics by HPLC analysis. Two antioxidants probucol and Vc were used as comparison. Fucoidan F-C with low molecular weight 2,000-8,000 and sulfate content 24.3% had much stronger protective antioxidant effects than other four Fucoidan fractions on both hydrophilic radical AAPH and lipophilic radical AMVN induced LDL oxidation. In AMVN induced LDL oxidation system, the other four Fucoidan fractions with molecular weight of 200,000 and 20,000 were able to modify the kinetics of LDL oxidation with the similar efficiency. However, the highly sulfated Fucoidan fraction L-B with molecular weight 20,000 was completely ineffective in protecting LDL from AAPH induced oxidation, whilst it effectively suppressed the oxidation of LDL induced by AMVN. The different protective antioxidant effects of Fucoidans on AAPH and AMVN induced LDL oxidation were due to their various structures and properties, and their capacities to interact with the different sites Fucoidans reacted on LDL.