Abstract: There were significant differences of infection ability of Beauveria bassiana on different target insect. Toxicity test was carried out using four different origin strains of B. bassiana against 2nd instar larvae of diamond back moth Plutella xylostella and cotton bollworm Helicoverpa armigera. Conidia germination and infection on the larval body surface of four strains were observed by scanning electron microscopic. We determined the growth rate, sporulation and extracellular enzyme activity of these strains. The results showed that the growth rate and enzyme activity of strain HFW-05 were higher than those of the other three strains. The corrected mortality of strain HFW-05 to diamond back moth was 87.1% at 6 d after inoculation. While the other three strains were about 30% to diamond back moth. The corrected mortality of four strains was lower than 20% for cotton bollworm after 6 d infection. The scanning electron microscopic observation results showed that strain HFW-05 directly penetrated diamondback moth epidermal after germinated or produced a shorter germ tube after 12 h infection. Mycelium failed to penetrate host cuticle but grew to break as blastospores after 36 h infection. For other treatments, scanning electron microscope results showed that conidia failed to penetrate host cuticle by cuticle infection, but affixed to the growth on the cuticle surface of host, and grew to break as blastospores with longer growth cycle than conidia of strain HFW-05. The results indicated it was not enough to evaluate and screen high virulent strains purely based on germination rate, conidia production or enzyme activity.