Abstract: Mamestra brassicae nucleopolyhedrovirus (MabrNPV) is the most important pathogenic microorganisms against cabbage moth. Major factors influencing mass multiplication of MabrNPV in vivo in the cabbage moth larvae and the role of these factors were studied by the orthogonal method. Factor interactions and the optimal conditions for mass production of MabrNPV were also investigated. The factors influencing MabrNPV yield were ranked according to their significance: larval age at virus treatment, interaction between larval age at virus treatment and virus harvest time, interaction between virus dosage and larval age at virus treatment, interaction between virus dosage and virus harvest time, virus dosage, virus harvest time, and temperature. Larval age at virus treatment and its interaction with virus harvest time significantly affected the production of MabrNPV. The optimal production conditions were: harvest virus at the 9th d post treatment of 12 d old larvae with 5×10² polyhedral inclusion bodies (PIB) per larva at 25 ℃. Under these conditions, MabrNPV production was 1.921×10⁹ PIB per inoculated larvae. The whole production cycle of MabrNPV lasted 21 d starting from larvae hatching to virus harvest. The reproduction rate of PIB was 3.842×10⁶ times.