The antioxidant activity in vitro of onion flavonoids was investigated by measuring its scavenging activity on DPPH, nitrite, its chelating activity on Fe2+, its inhibitory effect on peroxidation of yolk lipoprotein and oil. The results show that onion flavonoids posses strong chelating activity, good DPPH radical scavenging activity and nitrite scavenging activity, significant inhibitory effect on peroxidation of yolk lipoprotein and oil, namely onion flavonoids have good antioxidant activity in vitro. 