Abstract: Tropic Ageratum (Ageratum conyzoides) ,a common weed in south China,was reported to have allelopathic potentials in our previous paper. In this paper, the allelochemicals from aerial parts of the herb were isolated and identified. Their allelopathic potentials were investigated. Two compounds,which were chromatographed by TLC and HPLC were obtained in pure form. They were identified to be 6,7-dimethoxy-2,2-dimethylchromene (ageratochromene) and stigmastra-5,22-dien-3β-ol by means of IR,NMR(1H,13C) and MS. Bioassay results show that the ageratochromene was highly inhibitory to the seedling growth of the acceptor plants. The treatment with 500 ppm ageratochromene inhibited the seedling growth of barnyardgrass (Echinochloa crusgalli) and radish (Raphanus sativas) up to 100 % and 90 %, respectively. Treatment with 25 × 10-6ageratochromene had statistically significant inhibition to the seedling growth of barnyardgrass and the shoot growth of radish.