Abstract: By the method of baiting, 18 isolates were obtained, and further identificatied as Trichoderma viride.The antagonistic tests of all these isolates to 7 strains of Alternaria alternata from diseased tobacco leaves from different area of Yunnan were carried out by pairing culture on PDA.The results showed that 1 isolated (Tv-1)was strong antagonistic.The examination of antagonistic mechanism under optical microscope and transmission electron microscope showed Tv-1 hyphae growed quickly.Its hyphae could adhere to, coil around the hyphae of pathogen. Its finger-shaped haustorium penetrated into the hyphae of Alternaria alternat. Tv-1 could also secret antibiotic substance to make the pathogan hyphal plasma contracted, vacuolated and disrupted.