Abstract: A laboratory experiment was conducted to study the parasitic and lethal effects of Trichoderma longibrachiatum conidia suspension on Heterodera avenae cysts. Different concentrations (1.5×10⁵-1.5×10⁷ cfu·mL^-1) of T. longibrachiatum conidia suspension had strong parasitic and lethal effects on H. avenae cysts, and the effects differed significantly among the different concentrations. When treated with the T. longibrachiatum conidia suspension at a concentration of 1.5×10⁷ cfu·mL^-1, 96.7% of the H. avenae cysts were parasitized by the conidia at the 18th day, and the hatching rate of the cysts was inhibited by 91.2% at the 22nd day. The microscopic observation showed that at the initial parasitic stage, T. longibrachiatum conidia suspension adhered or parasitized on the cyst surface, germinated a large number of hyphae, and grew on the cyst surface, making the development of cyst embryo stopped and the contents in cysts flocculated, and even, some cysts started to deform, and small dark brown vacuoles formed on the cyst surface. At the later parasitic stage, the cysts were penetrated by dense mycelium, cysts were broken, their contents exosmosed, and the mycelium on the integument of some cysts produced conidiophores, on which, conidium were adhered or parasitized. It was considered that T. longibrachiatum could be used as a potential high-efficient bioagent to control the occurrence and damage of H. avenae.