Chitosan from marine crab shell with a degree of deacetylation of 0.89 was sulfated using chlorosulfonic acid/dimethylformamide under semi-heterogeneous conditions.The resulting chitosan polysulfate exhibits high solubility in common organic solvents and, more importantly, dissolves well in water.Characteristic absorptions in the IR spectrum at 810 and 1240 cm -1 , due to sulfo groups, were assigned to C-O-S and S=O stretch bonds stretching, respectively.The elemental analysis showed C, H, N and S contents of 15.03%, 4.08%, 2.84% and 14.46%, respectively, with degree of substitution (ds.) of 1.45.The chitosan polysulfate was separated by gel filtration chromatography using Sepharose CL-6B to provide three different products with average molecular weights of 6.6, 3.5 and 1.8 x 10 4 daltons, follow with purification by FPLC.All three chitosan polysulfate preparations revealed strong anticoagulant activities.Moreover, it was shown that they inhibited Factor Xa and thrombin activity.This mechanism is the same as heparin.All results indicated that chitosan polysulfate modified by using semi-heterogeneous conditions with potent anticoagulant activity has been synthesized.