Kangen-karyu (KGK) is a herbal formula created under the theory of traditional Chinese herbal medicine to invigorate the blood and dispel blood stasis. It contains 6 herbs: peony root, cnidium rhizome, safflower, cyperus rhizome, saussurea root (JP XIV), and Salvia miltiorrhiza root. The present study has been conducted to evaluate the in vivo anti-thrombotic activity of KGK using normal mice. Three consecutive days of oral administration of KGK to mice significantly extended tail-bleeding time and suppressed ex vivo platelet aggregation, while it did not extend the prothrombin time of plasma. It was revealed that the anti-thrombotic effects of KGK did not depend on the downregulation of the coagulation system, but depended in part on the inhibition of platelet aggregation. These results explain one of the pharmacological activities of KGK to invigorate the blood and dispel blood stasis.