The antiplatelet and antithrombotic activities of Korean Red Ginseng (KRG) were examined on rat carotid artery thrombosisin vivo, and platelet aggregationin vitro andex vivo. Administration of KRG to rats not only prevented carotid artery thrombosisin vivo in a dose-dependent manner, but also significantly inhibited ADP- and collagen-induced platelet aggregationex vivo, while failed to prolong coagulation times such as activated partial thromboplastin time (APTT) and prothrombin time (PT), indicating the antithrombotic effect of KRG might be due to its antiplatelet aggregation rather than anticoagulation effect. In line with the above observations, KRG inhibited U46619-, arachidonic acid-, collagen- and thrombin-induced rabbit platelet aggregationin vitro in a concentration-dependent manner, with IC50 values of 620±12, 823 ±22, 722±21 and 650±14 μg/mL, respectively. Accordingly, KRG also inhibited various agonists-induced platelet serotonin secretions as it suppressed platelet aggregation. These results suggest that KRG has a potent antithrombotic effectin vivo, which may be due to antiplatelet rather than anticoagulation activity, and KRG intake may be beneficial to the individuals with high risks of thrombotic and cardiovascular diseases.