The scavenging activities，stabilities and hemi-inhibitory concentrations(IC50) of 4 flavonoids,including rutin，vitexin，kaempferide and hyperoside to DPPH free radical were investigated by UV-Vis spectra，by using natural antioxidants ascorbic acid as the comparison.The relations of the flavonoids structures with their antioxidant activities were also discussed.The results indicated that different antioxidants have different balance time in scavenging the DPPH free radical，in which rutin need the longest time.Four flavonoids and ascorbic acid have scavenging effect to DPPH free radical，and the dose-effect relationship exists.The DPPH free radical scavenging abilities of these five antioxidants were as follows:hyperoside＞ascorbic acid＞rutin＞kaempferide＞vitexin.The analysis on the structures of four flavonoids showed that the pyrocatechol hydroxy group of B ring was necessary for the antioxidant activities of flavonoids compounds.The hydroxymethylation of -OH at B ring and the glycosylation of -OH at A ring were not favorable for antioxidant activities.The glycosylation of 3-OH at C ring could increase the antioxidant activities of flavonoids,in which the antioxidant activity of monloglycoside(e.g.hyperoside) is better than that of diglycoside(e.g.rutin).