Abstract: Six main allelochemicals, precocene Ⅰ, precocene Ⅱ, 3, 3-dimethyl-5-tert-butylindenone, β-caryophyllene, α-bisabolene and fenchyl acetate were isolated and identified from the volatile oil of a South China important weed—Ageratum conyzoides by means of column chromatography. Allelopathic effects of allelochemicals and the volatile oil on radish, tomato and mungbean were investigated through modes of volatilization, leaching (Saturated aqueous solution) and degeneration in soil. The results show that precocene Ⅰ, precocene Ⅱ, 3, 3-dimethyl-5-tert-butylindenone,β-caryophyllene (only volatilization mode) were highly inhibitory to the seedling growth of the acceptor plants, but β-caryophyllene (saturated aqueous solution), α-bisabolene and fenchyl acetate did not inhibited the seedling growth of acceptor plants. However, allelopathic effects of mixed solutions of precocene Ⅱ, β-caryophyllene and α-bisabolene or fenehyl acetate were significantly intensified,while the allelopathic effect of the volatile oil of A. conyzoids is also more intensive than that of other respective allelochemicals. It was revealed that synergistic action existed among allelochemicals of A. conyzoids