The protective effect of total flavonoids Astragalus (TFA), one of the natural anti-oxide agent isolated from Astragalus monghlious, on DNA strand breaks induced by the hydrogen peroxide and gamma rays has been investigated with the fluorometry assay of DNA unwinding (FADU). The results showed that the TFA has different degree of protective effect on DNA strand breaks caused by hydrogen peroxide and gamma rays respectively. It also showed some protective effect on DNA strand breaks induced by these two damage factors,when the concentration of TFA were 0.4 g/L and 0.6 g/L, respectively. When its concentra-lions were increased to 0.8 g/ L and 1.2 g/L, TFA could effectively protect DNA from hydrogen peroxide and gamma rays damages. Compared the protective effect of TFA on DNA strand breaks induced by hydrogen peroxide and gamma rays, the former was better. The protec-live mechanism of TFA on DNA strand break could be related to savenging free radicals.