The oriental armyworm Mythimna separata (Walker) is a major cereal crop pest, causing severe economic losses worldwide every year. Yet, few studies documented the biological control of M. separata using natural enemies such as egg parasitoids of the genus Trichogramma. To evaluate the possibility of biological control of M. separata eggs at various ages (0-, 1-, 2-, and 3-day old), we compared under laboratory conditions the parasitism, emergence, development, and female progeny (sex ratio) of five Trichogramma species indigenous in China: Trichogramma dendrolimi Matsumura, T. chilonis Ishii, T. ostriniae Pang & Chen, T. leucaniae Pang & Chen, and T. japonicum Ashmend. All five Trichogramma species parasitized M. separata at all egg ages but showed a preference for younger eggs. T. dendrolimi parasitized 55.1, 36.6, 19.3, and 14.3 hosts in 24 h on 0-, 1-, 2-, and 3-day-old eggs, respectively, and it exhibited the highest parasitism and suitability on all egg ages. T. japonicum showed the lowest parasitism rate and the poorest host acceptance. T. ostriniae, T. leucaniae, and T. chilonis presented similar rates of adult emergence and sex ratio for all M. separata egg ages. T. ostriniae developed significantly slower on 2- and 3-day-old M. separata eggs. With the exception of T. leucaniae, all species showed no difference in adult emergence at all egg ages. T. dendrolimi was identified as the best for biological control of M. separata. The present study provides valuable information for future development of Trichogramma species in biological control programs targeting M. separata as pest.