In this study, we investigated the effect of chlorogenic acid, a phenolic acid, on collagen (10 μg/ml)-stimulated platelet aggregation. Chlorogenic acid dose-dependently inhibited collagen-induced platelet aggregation, and suppressed the production of thromboxane A2 (TXA2), an intracellular Ca-agonist as an aggregation-inducing autacoidal molecule, which was associated with the strong inhibition of cyclooxygenase (COX)-1 in platelet microsomes having cytochrome c reductase activity. In addition, chlorogenic acid increased significantly the formation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), intracellular Ca-antagonists as aggregation-inhibiting molecules. These results suggest that chlorogenic acid has antiplatelet activity through the reduction of TXA2 and the increase of cAMP and cGMP levels. Therefore, our data demonstrate that chlorogenic acid is a potent inhibitor of collagen-stimulated platelet aggregation, and may be a crucial tool for a negative regulator during platelet activation in thrombotic diseases.