Abstract: To clarify the antifungal spectrum of Trichoderma longibrachiatum and the probable mechanism of T. longibrachiatum against plant pathogens, confrontational incubation and microscopic observation were used to determine the antifungal spectrum, and to explore the probable mechanism of T. longibrachiatum against twelve kinds of plant pathogens in vitro. The results of confrontational incubation experiment showed that T. longibrachiatum had the greatest magnitude of inhibition against the plant pathogens of Valsa ceratosperma and Rhizoctonia solani in vitro. The inhibitory rates of T. longibrachiatum against the plant pathogens of V. ceratosperma and R. solani were 86.4% and 72.5% respectively. It was further noted that T. longibrachiatum colony covered the whole colony of V. ceratosperma and the edge of R. solani colony, produced large numbers of spores on in whole colony of V. ceratosperma and the edge of R. solani colony 3 d after inoculation. Also the antagonism coefficient of T. longibrachiatum against V. ceratosperma and R. solani plant pathogens were ranked as Ⅰ and Ⅱ, respectively. However, the inhibitory effect of T. longibrachiatum on Bipolaris triticicola plant pathogens was weak, and the inhibitory rate was 27.5%. Meanwhile the antagonism coefficient of T. longibrachiatum against B. triticicola was grade Ⅲ. Analysis of antifungal mechanism of T. longibrachiatum showed that T. longibrachiatum produced inhibition zone, competed for nutrition and spaced with plant pathogens, and showed mycoparasitic behavior against plant pathogens when in contact with each other. Then, T. longibrachiatum mycelia covered the colonies of plant pathogens, grew on the surface and occupied the space of plant pathogens after contact with twelve pathogens. Plant pathogens mycelia appeared twisted after contact with T. longibrachiatum, and twisted part were so obviously constricted that protoplast of pathogen hyphae cell and mycelia eventually spilt and ruptured. T. longibrachiatum strains had different inhibitory effect on the growth of twelve kinds of plant pathogens which determined the probable mechanism of T. longibrachiatum against plant pathogens.