Antithrombotic activity of a protease purified from a medicinal mushroom, Ganoderma lucidum, has been evaluated platelet aggregation in vitro and pulmonary thrombosis in vivo. The purified protease exhibited concentration dependent inhibitory effects on platelet aggregation induced by ADP (adenosine diphosphate), with an IC(50) value of 2.4 mg/mL. The purified protease protected mice against thrombotic death or paralysis induced by collagen and epinephrine in a dose dependent manner when administered orally. It produced a significant inhibition of thrombotic death or paralysis at 60 µg/kg body weight, while aspirin produced a significant inhibition of thrombosis at 10-20 mg/kg body weight. The purified protease also has showed fibrinolytic activity and alters coagulation parameters such as activated partial thromboplastin time (APTT), and thrombin time (TT) in rat platelet. These results suggested that the antithrombotic activity of Ganoderma lucidum protease might be due to antiplatelet activity rather than anticoagulation activity.