The present study was undertaken to investigate the effects of Moutan Cortex Radicis extracts and paeonol, a major component, on rabbit platelet aggregation and thromboxane (TX) B₂ formation. Moutan Cortex Radicis methanol and butanol layers (100 μg/mL) showed the weak inhibitions, and water layer (100 μg/ mL) had no effect on the collagen-induced platelet aggregation. Whereas, hexane and EtOAc layers potently inhibited the collagen (3 μg/mL)-induced platelet aggregation with the IC_(50) values of 10.9±1.0 and 31.5±0.8 μg/mL, respectively. Paeonol isolated from the hexane-acetone layer specifically inhibited the collagen-induced platelet aggregation with the IC_(50) value of 113.1 ± 0.9 μM, whereas it had little effects on the other agonists such as AA-, thrombin-, A23187- and thapsigargin-induced platelet aggregations. Paeonol also potently inhibited the collagen-induced TXB₂ formation in rabbit platelet in a concentration-dependent manner. These results suggest that paeonol may inhibit rabbit platelet aggregation by interfering with an essential step in collagen-induced platelet activation and TXA₂ formation. Paeonol may be a promising candidate for an antiplatelet agent.