This study was focused on the purification, characterization and promotion mode of an anticoagulant polysaccharide from Hizikia fusiforme. The anticoagulant crude polysaccharide(HF 0) was obtained by using hot water extraction at 100oC for 3 hrs after homogenizing desalted Hizikia fusiforme. The anticoagulant polysaccharide(HF 2 3 1a) was purified from the crude extract(HF 0) through stepwise gradient ethanol precipitation(HF 2), DEAE Toyopearl 650C(HF 2 3), Sephadex G 75(HF 2 3 1), Sepharose CL 6B(HF 2 3 1a) chromatography and HPLC to homogeneity. HF 2 3 1a was estimated at 5.3$ imes$105 Da molecular weight and composed of fucose(51.92%), galactose(19.34%), mannose(13.92%), xylose (7.14%), arabinose(3.95%) and rhamnose(3.78%), and comprimised 29.7 % sulfate residue. The sulfated anticoagulant polysaccharide from HF 2 3 1a was proposed to inhibit via the intrinsic pathway and common pathway in the blood coagulation. The HF 2 3 1a exhibited the anticoagulant activity by activating an antithrombin III and the activity depended on the concentration of HF 2 3 1a. Acute toxicity of HF 2 in mice was not detected. Only 14 of 33 control mice(11.4%) that had taken saline survived for 30 min after injecting thrombin(100 NIH unit/ml).