Abstract: Daphnetin is one of the constituents of Daphne koreana Nakai. A Carotid-jugular extracorporeal shunt was made in the rat. A 5 cm length of silk thread was placed in the shunt. Fifteen rain after the restoration of blood flow in the shunt, the wet weight of thrombus developed on the thread was measured.The thrombus weight was significantly reduced when daphnetin was given ⅳ 40 or 80 mg/kg. The thrombus weight of control group was 24.4±3.8 mg, and that of the medicated group was 16.4±2.2 mg or 11.2±1.8mg. The inhibitory rate was 33% or 54%.In in vitro studies, ADP induced rabbit platelet aggregation was inhibited by daphnetin (0.025～0.1 mg/ml). Intravenous injection of daphnetin at a dose of 40 or 80 mg/kg inhibited rabbit platelet aggregation.Intravenous injection of dephnetin at a dose of 40 or 80 mg/kg reduced rat platelet adhesion. Daphnetin 40 or 80 mg/kg ip in mice prolonged the clotting time of blood. Results also indicate that neither the concentration of fibrinogen nor the blood viscosity was affected by intravenous injection of daphnetin.