Caffeic acid (CA), one of the active constituents of Radix Salvia miltiorrhizae, exhibits antioxidant and anti-inflammatory activities. However, few studies have assessed the ability of CA to inhibit platelet mediated thrombus generation in vivo. In this study, we investigated the antithrombotic effect of CA in mouse cerebral arterioles and venules using intravital microscopy. The antiplatelet activity of CA in ADP stimulated mouse platelets in vitro was also examined in attempt to explore the underlying mechanism. Our results demonstrated that CA (1.25-5 mg/kg) significantly inhibited thrombus formation in vivo. In vitro, CA (25-100 μM) inhibited ADP-induced platelet aggregation, P-selectin expression, ATP release, Ca(2+) mobilization, and integrin αIIbβ3 activation. Additionally, CA attenuated p38, ERK, and JNK activation, and enhanced cAMP levels. Taken together, these data provide evidence for the inhibition of CA on platelet-mediated thrombosis in vivo, which is, at least partly, mediated by interference in phosphorylation of ERK, p38, and JNK leading to elevation of cAMP and down-regulation of P-selectin expression and αIIbβ3 activation. These results suggest that CA may have potential for the treatment of aberrant platelet activation-related diseases.