Abstract: The effect of the defibrase of Agkistrodon acutus venom on blood coagulation system in rabbits both in vitro and vivo was reported in this paper.Our experiments showed that the defibrase had the same effects on the blood coagulation in vitro as thrombin did.The defibrase could transfer the plasma fibrinogen into fibrin.The later was not cross-linked.The principle was able to degrade the α-chain of the fibrinogen monomers,therefore,the thrombosis could not occur.In vivo,the defibrase remarkably prolonged whole blood coagulation time,one-stage prothrombin time and thrombin time.The plasma fibrinogen level in the blood of rabbits was decreased after injection,but there were no effects on the factor Ⅴ,Ⅶ,Ⅻ and platelet facfor level.The Ac-globulin dissolution time was shortened significantly.The experiment both in vitro and in vivo suggests that the defibrase contained both the properties of the fibrinolytic principle and anticoagulant principle.