Onion (Allium cepa L.) production in Korea has increased gradually over the past 15 years, placing second in food consumption survey with 20.6 g daily intake in 2006. Onions, used as an ingredient in many dishes and accepted by almost all traditions and cultures, have been reported to have a range of health benefits which include anticarcinogenic, antiasthmatic, antibiotic, and antioxidative effects. These effects may be attributable to a powerful flavonoid pigment-containing compounds, such as quercetin and alk(en)yl cysteine sulphoxides (ACSOs). Although antiplatelet and antithrombotic activities of onion have been confirmed by many of in vitro or animal studies, only a few human intervention studies have been examined. The majority of human studies identified that onion improves some cardiovascular markers such as lipid profile and platelet coagulant. With regard to antioxidative effects, somewhat positive effects are confirmed through strengthening the resistance of oxidative DNA damage in lymphocyte and urine, while most studies failed to find inhibitory effects on LDL oxidation. The discrepancies among studies might be ascribed to producing area, processing methods of onion, dosage, subject characteristics, study duration, and measurement methods. In this review, we focused on the preventive effect of cardiovascular disease through onion consumption in human intervention studies.