The role of oviposition site selection by oriental armyworm Mythimna separata Walker in defence against natural enemies and the effect of color properties on the selection were investigated in laboratory and field. The predation and parasitism for eggs of M. separata were investigated in the habitats of wheat, corn and weed, by attaching the covered and exposed egg masses on the stems of green plant or on the stems of dead weed. The results showed that, in all types of habitats, the probability discovered by predator for exposed egg mass was significantly higher in comparison with the covered egg mass on the same type of plant, respectively. In wheat and weed habitats, the predation rate for exposed eggs was significantly higher than that for covered eggs on green plants. The predation rate for the exposed eggs on green plant, in the three types of habitats, was significantly higher than that covered eggs on dead weed respectively. However, no significant differences were found in the predation rate between the covered eggs and exposed eggs on dead weed stems in weed habitat and corn habitat. No parasitoid was found from the covered or exposed egg masses in the experiments. The results suggested that both covered site and dead plant were critical components for the successful defence against predators. The survival time of neonate larvae of M. separata without any food supply was significantly longer than that of beet armyworm, Spodoptera exigua, which lays eggs on the host plants suitable for offspring development. Our results support the refuge theory′ for M. separata that females provide enemy-free space for the offspring. In the choice test for oviposition preference among millet straw, wheat straw, green wheat plant, corn plant and wax paper, 87.42% of the eggs were laid on the millet straw. The number of eggs on other substrates almost can be neglected. The oviposition preference on different colour paper revealed that 49.69% and 36.73% of the eggs was laid on the straw-yellow paper and yellow paper, respectively, which were significantly more than these on green paper. By comparing the spectrum properties of different substrates, it was found that millet straw, straw-yellow paper and yellow paper were similar in the reflecting rate at different wavelength. The results indicated that spectrum properties were the indicators for the oviposition site selection by M. separata.