Abstract: The optimum ethanol extraction process of onion flavonoids extracts by using ultrasonic wave, and antioxidant activity in vitro of the extracts were investigated. Ethanol was used as the extraction solvent, flavonoids of extracts were used to measure the extraction. Optimum conditions of extracting ethanol flavonoids extracts from onion was obtained by orthogonal experiment. Reducing power and scavenging to hydroxyl free radical were used to determine the antioxidant activity of the extracts. Results showed that optimum extraction conditions were as follows:ethanol concentration 70% , ultrasonic time 5min, material/liquid ratio 1∶15 (m∶V) , extracting time 1.5h. Under these above optimum conditions, the extraction content could reach 40.17mg/mL. The reducing power of 0.5mg/mL ethanol extracts was equal to 0.5mg/mL V C . Ethanol extracts from onion on hydroxyl free radical scavenging rate increased with the concentration ( 0 ～ 1 . 2mg / mL ) raising , when the concentration was 1.2mg/mL, the scavenging rate was 32.42% . Experiments showed that onion flavonoids extract had strong antioxidant activity , which could be used as antioxidants or provide resources for the development of functional food.