Abstract: In order to understand the infection process of Beauveria brongniartii(Sacc.)Petch to Cryptorhynchus lapathi L. larvae, we observed the ultrastructural changes of body wall of C. lapathi larvae infected by B. brongniartii using scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). SEM observation indicated that, 12 hours after inoculation, spores adhered onto different areas of body wall of the C. lapathi larvae such as spiracles, sensilla surroundings and internode membranes. Twenty-four hours after inoculation, hypha or germ tubes of B. brongniartii penetrated into the integuments of C. lapathi larvae, and no appressoria was found. After inoculation for 90 and 96 hours, hypha penetrated out from the surface of C. lapathi and began to multiply rapidly. TEM observation showed that the penetration of hypha was the combination of mechanical pressure and histolysis. After inoculation for 24 hours, hypha penetrated into cuticula, and 36 hours after inoculation, a lot of hypha penetrated into the epidermis. After inoculation for 48 hours, epidermis deformed after the hypha penetrated into it and halos with low electron density could be observed at the front end of penetrated hypha. After inoculation for 60 hours, cuticula was separated from epidermis, a lot of hypha penetrated into the epidermis, organelles near hypha were dissolved, cell nucleuses were destroyed, and flocculent solutes appeared around the hypha. Those phenomena, reported for the first time in our study, confirm that enzymatic action was involved in the process of B. brongniartii intruding into C. lapathi larvae. After inoculation for 72 hours, the cell structure was loosened and became abundant vacuolus. After inoculation for 84 hours, hypha massively self-reproduced by non-septa or one-septa or two-septa ways in blood cavity. After inoculation for 96 hours, hypha penetrated out from cuticula.