Relationships among survival rate of Microplitis sp, developmental duration of Microplitis sp in bodies of Spodoptera litura larvae , exposure time of host larvae to Spodoptera litura nuclear polyhedrosis virus(SlNPV) and concentration of inoculation virus were studied. Meanwhile, efficiency of Microplitis sp’s transmitting virus was also determined in this study .The results showed that: 1) SlNPV didn’t affect significantly developmental period of parasitic larvae; 2) Parasitic wasps in bodies of Spodoptera litura larvae could complete their development before their hosts died of virus infection. The survival proportion of parasites varied with the concentration of inoculation virus and the exposure time of host larvae to virus. Spodoptera litura larvae were inoculated with SlNPV after they have been parasited by the wasps, the percent of wasps which could complete their development, increase as the parasitizing time goes, and decrease with the increase of inoculation virus concentrations. However, the exposure time to virus was the primary influcing factor; 3) The female parasitic wasps which developed or oviposited in bodies of virus-infected host larvae , and the parasitic wasps which were manually contaminated their ovipositor with virus suspension, could all carry a number of SlNPV; 4) Adults of Microplitis sp could transmit the virus among host larvae through oviposition. Although parasitic wasps oviposited in bodies of virus- infected host larvae , they must certainly carry this sort of virus , then they could averagely transmit the virus to 2.14 healthy host larvae. Meanwhile,  those parasitic wasps which developed in bodies of virus-infected host larvae could averagely transmit the virus to 2.45 healthy host larvae. 5) The ovipositors of parasitic wasps might be contaminated with SlNPV by two methods, i.e., emerging their cocoons in the virus suspension or raising adults of wasp on mixture of virus and honey , and their efficiency of transmitting virus increased with the increase of concentrations of inoculation virus; parasitic wasps from the first method, i.e., emerging cocoons could transmit infective doses of the virus to an average of 1.45 healthy host larvae, and wasps from the second method, i.e., raising wasps on mixture could also transfer virus to an average of 0.94 larvae.