Many clinical trials have demonstrated the beneficial effects of garlic (Allium sativum) on general cardiovascular health. Aged garlic extract (AGE) is known to display diverse biological activities such as in antioxidant, anti-inflammatory and anticancer activities. However, few studies have been directed on the effect of AGE on cardiovascular function. In this study, we aimed to investigate the effect of AGE and its components on platelet activation, a key contributor in thrombotic diseases. In freshly isolated rat platelets, AGE and its components have shown inhibitory activities on thrombin-induced platelet aggregation. These in vitro results were further confirmed in an in vivo platelet aggregation measurement where tail vein injection of garlic oil and S-Allylmercapto-cysteine (SAMC) significantly reduced thrombin and ADP-induced platelet aggregation. Potential active components for antiplatelet effects of AGE were identified to be SAMC and diallyl sulphide through agonist-induced platelet aggregation assay. These results indicate that aged garlic extract can be a novel dietary supplement for the prevention of cardiovascular risks and the improvement of blood circulation.