To obtain a full scale assessment of the influences of transgenic Bt crops on the oriental armyworm, Mythimna separata (Walker), larval growth and development, adult reproduction, and flight potential of M. separata were examined when newly hatched larvae continuously fed on artificial diet containing different concentrations of Cry1Ac toxin. The results showed that the mean head capsule width, and body length of the larvae fed with diets containing 0 (CK), 3, 6, 12, 24, 48 and 96 &amp;#956;g/g of Cry1Ac toxin for 6 d were significantly decreased as the concentration of toxin increased. Similarly, body weight, mortality, and the developmental period of the larvae treated for 18 d were negatively and significantly affected as the Cry1Ac dosage increased. The larvae fed with diets containing more than 12 &amp;#956;g/g of Cry1Ac toxin could not pupate, while the larvae fed with diets containing 0, 3 and 6 &amp;#956;g/g of Cry1Ac toxin could pupate and emerge normally. Furthermore, the pupal duration was extended, and the pupation rate decreased significantly as Cry1Ac concentration increased from 0 (CK) to 3, and 6 &amp;#956;g/g. Reproductive potential of the moths resulting from the larvae fed with diets containing 0, 3, and 6 &amp;#956;g/g of Cry1Ac toxin was also significantly different. Preoviposition period of adults was significantly prolonged, and the number of eggs laid per female was significantly decreased as the dosage of Cry1Ac toxin increased. Flight potential (including flight duration, distance, and velocity) of the 1-day-old moths was significantly decreased as the dosage of Cry1Ac toxin increased by tethered-flight test. All these results showed that the Cry1Ac had a significant impact on growth and development, reproduction and flight potential of M. separata, and this will help to improve the evaluation of ecological safety of Bt-crops.