Abstract: The strain of Xi Nong No.24 was separated from the soil collected in Qinling,Shaanxi province. It was indentified to be streptomyces,taking as object,by means of micro-pannikin, herbicidal activity had been measured on Amaranthus retroflexus L. by fermentaing,eluting and Pre-HPLC.The results of biological test showed that the fermentation products of No.24 had an intensive herbicidal activity against Amaranthus retroflexus L., and had an inhibition of 80.6% to taproot, an inhibition of 66.5% to caulis and an inhibition of 26.7% to bourgeon. Exchange liquor had an inhibition of 93.1% to taproot, an inhibition of 86.2% to caulis and an inhibition of 42.9% to bourgeon.Eight fractions were isolated from the fermentation products of No.24 strain by Pre-HPLC. The fraction No.4 had the most intensive herbicidal activity. The inhibiting rate of No.4 was 95.8%, 93.76% and 86.36% against the taproot ,caulis and the bourgeon of Amaranthus retroflexus L.respectively.