Platelets serve many biological functions, including a major role in the haemostatic process. But platelets also play a crucial role in the formation of arterial thrombosis, arteriosclerosis and other pathologic processes. Thus, there have been many studies to develop new antiplatelet agents from foods and plants for decades. Inthis study, inhibitory effects of the oriental onion (Allium fistulosum) on platelet aggregation were investigated using platelet rich plasma (PRP). Water extracts of oriental onion was separated into two fractions (Fraction I and Fraction II) by Sephadex G-150 column. Platelet aggregations were inhibited by total water extracts as well as Fraction I and II. IC50 value of Fraction I was much lower than that of Fraction II. Inhibitory effects of total water extracts of oriental onion on ATP release by PRP were also observed.