[Objectives]  To investigate the cold hardiness of the beet armyworm Mythimna separata (Walker) in order to provide a basis for predicting outbreaks of this pest. [Methods]  We measured the super-cooling point (SCP), freezing point (FP) and water content of armyworms at different developmental stages that were raised under different photoperiods. Armyworms overwinter as pupae and mature larvae in some regions, so pupae and mature larvae were measured at 0, 5 and  10℃, respectively. [Results]  1. The SCP, FP and water contents of pupae were lowest, and there were significant differences in the SCP, FP and water content of different development stages (P<0.05), but not different photoperiods. 2. The survival rate of larvae and pupae varied greatly with temperature. Mortality increased as temperature decreased from 10℃ to 0℃ at intervals of 5℃, regardless of developmental stage. The mortality of pupae was higher than that of mature larvae. The susceptibility of male and female pupae to low temperature differed; female pupae had higher mortality at lower temperatures when the exposure time was shorter. [Conclusion]  The cold hardiness of pupae was highest, and the sustained duration of low temperature influenced the sex ratio of pupae and the mortality of overwintering individuals, thus affecting the dynamics of the population in the following year. Photoperiod had no influence on the cold hardiness of armyworms.