In order to understand the compsition and structure of herbicidal component of Pythium aphanidermatum,the isolation and structural indentification were researched.The culture filtrate was extracted by ethyl acetate,petroleum,and chloroform with the same volume respectively and the activity of the crude toxin was bioassayed.The toxin was separated by using the method of thin layer chromatography(TLC),then the main fraction was separated by HPLC,and the structure was analyzed by the sepctrum of IR,13C-NMR and 1HNMR.The results showed that the ethyl acetate extracts had the strongest herbicidal activity.Using the method of TLC,the bioassay results showed that the extracts with Rf 0.19 had the strongest effect on weeds and the inhibition to Digitaria sanguinalis and Amaranthus retroflexus reached five levels,and the component was proved to be dimethyl o-phthalate from the spectrum of IR,13C-NMR and 1HNMR,which was one of the components from the toxin,and it had herbicidal activity. In order to understand the compsition and structure of herbicidal component of Pythium aphanidermatum, the isolation and structural indentification were researched. The culture filtrate was extracted by ethyl acetate, petroleum, and chloroform with the same volume respectively and the activity of the crude toxin was bioassayed The toxin was separated by using the method of thin layer chromatography (TLC), then the main fraction was separated by HPLC, and the structure was analyzed by the sepctrum of IR, 13C-NMR and 1H NMR. The results showed that the ethyl acetate extracts had the strongest herbicidal activity. Using the method of TLC, the bioassay results showed that the extracts with Rf 0.19 had the strongest effect on weeds and the inhibition to Digitaria sanguinalis and Amaranthus retroflexus reached five levels, and the component was proved to be dimethyl o-phthalate from the spectrum of IR, 13C-NMR and 1H NMR, which was one of the components from the toxin, and it had herbicidal activity.