Abstract: The antifungal mechanism of Trichoderma viride strain TR-8 against Fusarium oxysporum was studied. When TR-8 and F. oxysporum strain FO-G1 were dual-cultured on PDA plate, TR-8 grew faster than FO-G1. After the two kinds of mycelia contacted, mycelial growth of FO-G1 stopped. Later, TR-8 continued to grow, covered FO-G1 colony and produced large number of conidia. Inhibition ring was not found. The mycoparastism of TR-8 against FO-G1 was observed. It was revealed microscopically that at first TR-8 mycelia grew to FO-G1, then developed in parallel or penetrated FO-G1 mycelia, eventually FO-G1 mycelia collapsed. After TR-8 was fermented 7 d, the activities of 1,3-β-glucosidase and chitinase were 20.15U/ml and 11.67U respectively. Low antibiosis of TR-8 was detected. In vitro, fermentative liquid, even which was not diluted, couldn't inhibit both the mycelial growth and spore germination of FO-G1. The volatile substances secreted by TR-8 could inhibit spore germination, with inhibition rate reached 75.06%.