Monochamus alternatus microsporidium is a newly discovered pathogen whose scientific name is Nosema alternatus. The pathogen has a strong pathogenicity to the longhored beetles. According to the field investigation, about 7.7% of the real pathogens for naturally dead longhorned beetles were microsporidiums. The Results of bioassey showed that the mortality of longhorned beetle larvae is 44.1%, at 25 ℃ after 20 days. The microsporidium possesses the ability of vertical dissemination and hopefully to be an effective natural organism to control the M. alternatus beetles.