The antithrombin effects of the sub-fractionated fucans with different molecular weights and sulfate contents, which were prepared from a fucan sulfate isolated from the brown seaweed Ecklonia kurome, were examined for their abilities to inhibit thrombin-fibrinogen reaction and amidolytic activity of thrombin, and to bind to fibrinogen. The inhibitory effects of the fucans on both fibrinogen clotting by thrombin and amidolysis of the protein in the presence of heparin cofactor II were improved with increase in their molecular weights and reduced with decrease in their sulfate contents. The binding abilities of the fucans with almost the same sulfate content to fibrinogen were unchanged independently of their molecular weights, although the ability diminished with decrease in the sulphate content. These results suggest that heparin cofactor II-mediated antithrombin activity of the fucan sulfate is dependent on both its sulfate content and molecular weight, and also that the inhibitory effect of the polysaccharide on fibrinogen clotting by thrombin may be attributable to the steric hindrance by its binding to fibrinogen.