Abstract: This paper deals with results of our investigation on the factors affecting the propagation of the nuclear polyhedrosis virus (NPV) of the polar looper,Apocheima cinerarius Erschff. The concentration had a conspicuous effect on the propagation of the nuclear polyhedrosis virus.Among the concentration tested,the 1.1×10⁶-1.1×10⁷ PIB/ml was found to be the best on early four instars average 2.42-2.64×10⁹ PIBs/larvae.When the larvae of poplar looper were fed with the poplar leaves contaminated with polyhedral inclusion bodies (PIB) of the virus,the mortality rate of the infected larvae was depended on concentration of the virus suspension and laral instar. The amount of propagated NPV was found to be proportional to the concentration of the inoculum suspension.The lag of the death of larvae inoculated with NPV was influences by the temperature and suspensioncon centration of the polyhedral virus.The propagation of the virus was inhibited at 35℃ and the optimal temperature was 22—27℃.The NPV would propagaten better in early four-instar larvae than in other instars on an average of 2.42×10⁹ PIBs/larvae. The propagation of field indicated that optimum suspension contration was 2.5×10⁶ PIBs/mu to second-instars and third-instars.Dry proparation of PIB of nuclear polyhedrosis virus of Apocheima cinerarius by precipitation with acetone from lactose suspensions indicated that the LC₍₅₀₎ values of 2.84×10~3 PIBs/ml for early third-instar larvae and 95% fiducial limit was 1.70×10³-4.96×10³ PIBs/ml.