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) $A_2$ formation. Moutan Cortex Radicis methanol and butanol layers (100 ${mu}g/mL$) showed the weak inhibitions, and water layer (100 ${mu}g/mL$) had no effect on the collagen-induced platelet aggregation. Whereas, hexane and EtOAc layers potently inhibited the collagen (3 ${mu}g/mL$)-induced platelet aggregation with the $IC_{50}$ values of 10.9${pm}$1.0 and 31.5${pm}$0.8 ${mu}g/mL$, respectively. Paeonol isolated from the hexane-acetone layer specifically inhibited the collagen-induced platelet aggregation with the $IC_{50}$ value of 113.1 ${pm}$ 0.9 ${mu}M$, 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 TXB 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 $TXA_2$ formation. Paeonol may be a promising candidate for an antiplatelet agent.