Abstract: Mikania micrantha,an exotic plant species, is an important weed in south China. The volatile oil of the weed species had significant biological activities on plants and their pathogens, particularly Pyricularia grisea. The results showed that the seeding growth of 6 plant species decreased obviously when exposed to increasing concentration (200, 400, 800, 1600 mg·L^-1) of the volatile oil. The fresh weight of all test plants decreased,and the emergence of all test plants delayed for 1～2 days under soil treatment (2500 g·hm^-2).The inhibitory effect of volatile oil of M. micrantha at its medium concentration was the most strong on Pyricularia grisea ;secondary on Fusarium oxysporum;but weak on Phytophthora nicotianae.The inhibitory rate was 53.38%, 28.66% and 18.69% for the three plant pathogens,respectively.