[Objectives]  Isaria javanica is an important entomopathogenic fungi that has been widely used to control various insect pests. The BE01 strain of I. javanica that is pathogenic to Hyphantria cunea was isolated from the surfaces of a cadavers of H. cunea to better understand its insecticidal effect and improve its use as a biological control agent for H. cunea. The process of infection by this strain and the host's associated antioxidant enzyme response were investigated. [Methods]  Scanning electron microscopy was used to observe third-instar H. cunea larvae infected by I. javanica BE01 conidia. Changes in levels of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in the bodies of H. cunea after inoculation were measured. [Results]  The infection process had several stages, including adherence of the conidia to the host cuticle, germination of the conidia 6 h after inoculation, development of appressorium after 24 h, hyphal growth on the body surface after 48 h, emergence of mycelium from the body and production of conidia after 96 h, and the entire body being covered by mycelium after 120 h. Activity of three antioxidant enzymes, SOD, POD and CAT, in larvae significantly increased after infection with I. javanica BE01 and peaked 48 h after infection. The activity of all three enzymes began to decrease after 48 h, and were lower than in the control group after 60 h. [Conclusion]  Conidia of I. javanica BE01 have high virulence and a rapid germination rate, which makes it difficult for H. cunea to resist infection. The I. javanica BE01 strain can quickly infect H. cunea larvae, and has the potential to develop into a new, highly effective biological control agent.