As the most potent natural anticoagulant known up to now, natural hirudin has been proved to exert a strong inhibition on the thrombin. It is extracted in the salivary glands of the leech Hirudinaria manillensis as the biological active material which composed of 65-66 amino alids a low-molecular-weight polypeptide. It can prevent the forming of arterial thrombosis after surgical operation, improve the blood circulation and haemodialysis, cicatrize the wound. In addition, hirudin has effect in lowering blood pressure and blood fat, expanding blood vessel.