In 2001-2003, an ultra low volume (ULV) Beauveria bassiana (Bb) oil formulation was used to prevent against Dendrolimus punctatus, and its effect on biological diversity (species richness, individual number, species diversity index and evenness index, and dominant index) was investigated. The results showed that there were 473, 392 and 266 species in non-control, Bb control and chemical control areas, respectively. The individual number ratio of natural enemies to pest in Bb control area (0.1049:1) was close to that in non-control area (0.1051:1), but much higher than that in chemical control area (0.0558:1). In Bb- and chemical control areas, the species richness and diversity decreased, but dominant index increased. There existed significant difference between chemical control and non-control areas, but no significant difference was found between Bb control and non-control areas. ULV Bb had no obvious effect on the natural enemies and non-target insects.