Abstract: The lethal effect of the conidia suspension of Trichoderma longibrachiatum against second stage juveniles of Heterodera avenae was observed under microscopic conditions and studied in vitro to preliminarily determine the potential and mechanism of the conidia suspension of T. longibrachiatum against H. avenae. Microscopic observation results showed that the conidia suspension of T. longibrachiatum adhered to or parasitized on the surface of second stage juveniles, even some parasitized nematodes were deformed at the initial stage. Later, the second stage juveniles were wrapped and the integuments were penetrated by a large number of hyphae germinated from the conidia suspension of T. longibrachiatum. Even the body of dead second stage juveniles was completely lysed. In vitro studies showed that different concentrations (1.5×10⁵-1.5×10⁷ cfu·mL^-1) of T. longibrachiatum (conidia suspension) had strong lethal and parasitic effects on the second stage juveniles of H. avenae, and significant differences existed among the treatments with different concentrations of T. longibrachiatum. In addition, the lethal and parasitic effects increased with increasing the T. longibrachiatum concentration. The mortality and corrected mortality of the second stage juveniles treated with the concentrations of 1.5×10⁷ cfu·mL^-1 were 91.3% and 90.4% after 72 hours, and the second stage juveniles were parasitized by 88.7% after 14 days. Therefore, the conidia suspension of T. longibrachiatum had a great lethal effect on H. avenae, and the strain could be considered as a potential bio-control agent against H. avenae.