Abstract: Flavonoids were extracted from Hawthorn seed under ultrasonic assistance, with ethanol as extraction solvent. The technical conditions were investigated based on single factor and orthogonal tests. Meanwhile, compared with ascorbic acid and rutin, the antioxidant activity of flavonoids was evaluated by scavenging DPPH radical, hydroxyl radical and method of Fe3+. The results showed that the optimal ultrasonic-assisted extraction condition of flavonoids from Hawthorn seed were shown as follows: ethanol volume fraction of 50%, material-to-liquid ratio of 1:25 (25 mL liquid per gram materials), ultrasonic power of 250 W, ultrasonic-assisted extracting time of about 40 min and extraction temperature at 60℃. The extraction rate of flavonoids reached 7.89%. The flavonoid extraction exhibited obviously ability of scavenging DPPH free radical and OH free radical, and showed a higher reducing capacity to Fe3+. The antioxidant activity increased with the increase of the concentration of purified flavonoids, which was superior to rutin but inferior to ascorbic acid.