Scavenging effects of total flavonoids of Astragalus (TFA), total saponins of Astragalus (TSA) and total polysaccharide of Astragalus (TPA) on superoxide anion radical and hydroxyl radical have been studied by ESR method. The results showed that all the three principles have stronger scavenging effect on superoxide anion radical than on hydroxyl radical. The intensity of scavenging effect of the three principles was TFA＞TSA＞TPA. It is suggested that the TFA and TSA are the main component in Astragalus mongholicus Bunge for protection against free radical damages.