In this study, we investigated whether the mixtures of onion water extract and aloe ethanol extracts have antiplatelet activities. The mixtures inhibited collagen- and thrombin-induced rat platelet aggregation in vitro. Additionally, the oral administration of the mixtures inhibited platelet aggregation induced by collagen ex vivo but not prolonged mouse tail vein bleeding time in vivo. These results suggest that the combination of onion and aloe extracts has a potential to be a preventive agent against platelet-mediated disorders.