Microplitis mediator (Haliday) is an important natural enemy to parasitize young larvae of some moths of Noctuidae (Lepidoptera). The results of field tests showed that in the middle region of Hebei province, when the daily average temperature was 21.5[degree]C and the daily average day length was 12 hours and 33 minutes, a few of M. mediator entered diapause. When the daily average temperature was below 17.9[degree]C and day length was shorter than 11 hours and 45 minutes, all of M. mediator entered diapause. The results of simulation test in the laboratory showed that when the temperature was between 17-26[degree]C and day length was 10-14 hours, the diapause rate significantly increased with the decrease of temperature and day length. High temperature counteracted the effect of short day length. When the temperature is 26[degree]C, photoperiod had no effect on diapause. Thus, lower temperature and shorter day length were the major factors to induce diapause of the parasitoid. The parasitoid accepted the signal for diapause at the young larvae stage and entered diapause at the pre-pupa stage (cocoon). All pre-pupae entered diapause only if the young larvae had been reared in the temperature and the photoperiod for diapause induction till mature larvae. The emergence rate and the parasitic ability showed no significant difference between adults emerged from the diapause cocoons stored in the temperature of 4[plus or minus]2[degree]C for 240 days and those from the non-diapause cocoons without cold storage. After 300 days cold storage, the emergence rate of the diapause cocoons could still reach to 81.4%. none