The possibility of Chungpesagantang, which has been recommended for stroke patients in Korean Sasang Constitutional Clinics, and its ingredients as a novel antithrombotic agent was evaluated. Chungpesagantang potently inhibited ADP and collagen-induced rat platelet aggregation in vitro as well as ex vivo in a dose-dependent manner. Puerariae Radix and Rhei Rhizoma potently inhibited ADP-induced rat platelet aggregation ex vivo. However, Puerariae Radix did not inhibit both in vitro ADP and collagen-induced rat platelet aggregations. Chungpesagnatang and its ingredients except Rhei Rhizoma did not affect certain plasma clotting times, such as APTT, PT, and TT. Chungpesagantang and its ingredients Raphani semen and Scutellariae Radix showed significant protection against death due to pulmonary thrombosis in mice.