Abstract: We examined the infection of Paecilomyces fumosoroseus via electron microscopy scan and pathogenicity of P. fumosoroseus conidia powder, conidia suspension liquid and conidia broth. Results show more conidia in acanthoid surface topography than in spinous surface topography. Conidia germinates earlier in spinous surface topography than in acanthoid surface topography and head area. Most conidia in pinous surface topography germinate 16 h after inoculation, whereas those in acanthoid surface topography germinate after 20 h, followed by those in head areas. Most conidia penetrate host cuticle with germ tubes in acanthoid surface topography and spinous surface topography of host while a few conidia does so with appressorium. Furthermore, infection velocity and pathogenicity are affected by inoculation method. Conidia from cultured broth infect hosts faster than those from suspension liquid and conidia powder. Pathogenic symptom of diamondback moths treated with conidia cultured broth is observed 44 h after inoculation, but those treated with conidia suspension liquid and conidia power is notable till after 60 h. LT50 of diamondback moth treated with conidia broth, conidia suspension liquid and conidia power are 3.18, 4.20 and 4.34 days respectively. Pathogenicity induced by conidia powder is strongest and mortality is 76.2% at 6 days after inoculation. In contrast, mortality from conidia suspension liquid and conidia broth at the same time is 59.8% and 58.5% respectively. Conidia power has the highest pathogenicity.