This study addresses the morphological changes of Trichoderma reesei HC-415 in the process for cellulase production in 30 L fermenter. It was found that the morphological changes of HC-415 could be divided into four periods in the process of fermentation. The hypha morphology during the first period showed that filamentous elongation with less branching and the enzyme production was almost at a stationary phase after vaccinations within 0~24 h. Next, the mycelial length and the number of branches increased, the stem became thick and the enzyme activity significantly increased after vaccination within 24~60 h. Then, mycelium with septa became short and thick, and this was accompanied by the appearance of many enlargement round nodules at the top of the branches after vaccination within 60~132 h. This was the period of rapid growth and peak stability of cellulase activity. Finally, the number of thick and short round mycelium with many enlargement nodules declined, accompanied by the gradual decline of enzyme activity after vaccination for 132 h. The results demonstrated that a tight correlation existed between the morphological characteristics and the cellulase production and the activity of Trichoderma reesei in liquid. Furthermore, the morphological characteristics can be used as an important indicator for the regulation of fermentation broth of maturity in the production of cellulase.