Geijibokryunghwan has a wide range of therapeutic applications, and some reports have indicated that it has protective activity against atherosclerosis, and more specifically stroke and myocardial infarction. A recent report showed that atherosclerotic plaque volume can be reduced by supplying Geijibokryunghwan extracts for several years. In this study, we used a component of Geijibokryunghwan, which has been used for the prevention of atherosclerosis in Korea for several years, and has proven to be useful in lowering the occurrence of cerebral infarction. In a preliminary study, we found that Geijibokryunghwan potently suppressed platelet aggregation induced by various agonists. In this study, we sought to explore the mechanism by which Geijibokryunghwan inhibits platelet aggregations.