In this study, we investigated the effect of egg yolk lipids (EYL) on collagen ($10;{mu}g/ml$)-stimulated platelet aggregation in vivo. Dietary EYL significantly inhibited collagen-induced platelet aggregation, in addition, increased the formation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), intracellular $Ca^{2+}$-antagonist as aggregation-inhibiting molecules, in collagen-stimulated platelets. These results suggest that EYL inhibits the collagen-induced platelet aggregation by up-regulating the cAMP and cGMP production. On the other hands, prothrombin time (PT) on extrinsic pathway of blood coagulation was potently prolonged by dietary EYL in vivo. These findings suggest that EYL prolongs the internal time between the conversion of fibrinogen to fibrin. Accordingly, our data demonstrate that EYL may be a crucial tool for a negative regulator during platelet activation and blood coagulation on thrombotic diseases.