Five Radix Astragali (RA) extracts were prepared and their antioxidant activities were measured in vitro using ABTS [2, 2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid diammonium salt)], DPPH · (1,1-diphenyl-2-picrylhydrazyl radical), reducing power, •O₂⁻ and •OH assays. Their chemical contents were then determined, including total phenolics, total flavonoids, total saponins and total sugars. The 1/IC₅₀ values of the various antioxidant assays were used to evaluate the level of antioxidant activity of the RA extracts (plot showing 1/IC₅₀ values vs. chemical contents), and the average R values (correlation coefficients) of total phenolics and total flavonoids were 0.762 and 0.638. In contrast, the average R values of the total saponins and total sugars were -0.0386 and -0.132, respectively. This large difference clearly demonstrates that the antioxidant effects of RA in vitro might be generally considered to be a result of the presence of phenolic compounds (including flavonoids) but not astragalosides and polysaccharides.