The possibility of Galla Rhois(GR) water extract as an antithrombotic agent was investigated. The effect of GR on platelet aggregation in human platelet-rich plasma(PRP) induced by collagen and ADP in vitro and coagulation parameters in a pathological model induced by endotoxin and hydrocortisone acetate(HA) in vivo were examined. In platelet aggregation assay, GR extract significantly inhibited platelet aggregation induced by collagen and ADP in a dose-dependent manner. GR extract significantly increased the number of platelet and shortened prothrombin time(PT) and activated thromboplastin time(APTT) as compared with the control in pathological model induced by endotoxin and HA. Also, GR extract significantly increased fibrinogen level as compared with the control in a pathological model induced HA. These results suggest that GR may be a promising antithrombotic agent.