Inhibitory mechanism of the anticoagulant polysaccharide purified from Codium fragile was investigated. The anticoagulant compounds (Cf-30-IV-4-ii, CF-30-IV) prolonged the clotting time at both activated partial thrombo-plastin time (aPTT) and thrombin time (TT). The Inhibition factor assay of instrinsic coagulation pathway in the blood showed that the anticoagulant polysaccharide (CF-30-IV-4-ii) inhibited other factors such as Ⅷ, Ⅸ, Ⅵ and Ⅷ of the coagulation cascade, which did not affect the lupus anticoagulant AB activity. In the thrombin inhibition pattern the CF-30-IV-4-ii did not directly influence the fibrine formation mediated by thrombin but af-fected the anticoagulant activity through the activation of antithrombin III. Base on these result, the anticoaglant polysaccharide (CF-30-IV-4-ii) was considered to inhibit serine pretense involved in the blood coagulation cascade through the enhancing antithrombin III activity. The residual effects of anticoagulant activity and antithrombosis were tested with ICR mice. The anticoagulant polysaccharide (CF-30-W) kept its anticoagulant activitv for 6 hrs with 100% survival at a dose of 150 mg/kg in the antithromboisis test. The anticoagulant effect of CF-30-RF in ex vivo was proportional to the concentration of intravenously injected dose up to 100 mg/kg.