Expression of insecticidal protein for transgenic Bacillus thuringiensis(Bt) cotton is unstable and related to nitrogen metabolism.The objective of this study was to investigate the relationship between leaf carbon nitrogen ratio(C/N) and insecticidal efficacy of two Bt cotton cultivars.C/N ratio and Bt protein content were both measured at peak square period and peak boll period respectively under 5-7 d high temperature and different nitrogen fertilizer rates on the Yangzhou University Farm and the Ludong Cotton Farm,China.All plants were grown in field.The results showed that the C/N ratio enhanced slightly and the Bt protein content remained stable at peak square period,but significant increases for the C/N ratio and decreases markedly for the leaf Bt protein concentration were detected at the peak boll period.The similar patterns at the two growth periods were found for the leaf C/N ratio and Bt protein content by different N fertilizer treatments.When nitrogen rate was from 0 to 600 kg ha-1,the C/N ratio was reduced by 0.017 and 0.006 for Sikang 1 and Sikang 3 at peak square period,compared to the 1.350 to 1.143 reduction for Sikang 1 and Sikang 3 at peak boll period,respectively.Correspondingly,the leaf Bt protein contents were bolstered by 2.6-11.8 and 26.9-36.9% at the two different growth periods,respectively.The results suggested that enhanced C/N ratio by high temperature and nitrogen application may result in the reduction of insectiocidal efficacy in Bt cotton,especially in peak boll period. Objective of this study was to investigate the relationship between leaf carbon nitrogen ratio (C / N) and insecticidal efficacy of two Bt cotton cultivars. C / N ratio and Bt protein content were both measured at peak square period and peak boll period respectively under 5-7 d high temperature and different nitrogen fertilizer rates on the Yangzhou University Farm and the Ludong Cotton Farm, China. All plants were grown in field The results showed that the C / N ratio increased slightly and the Bt protein content remained stable at peak square period, but significant increases for the C / N ratio and decreased markedly for the leaf Bt protein concentration were detected at the peak boll period. The similar patterns at the two growth periods were found for the leaf C / N ratio and Bt protein content by different N fertilizer treatments. WHEN nitrogen rate was from 0 to 6 00 kg ha-1, the C / N ratio was reduced by 0.017 and 0.006 for Sikang 1 and Sikang 3 at peak square period, compared to the 1.350 to 1.143 reduction for Sikang 1 and Sikang 3 at peak boll period, respectively. Correspondingly, the leaf Bt protein contents were bolstered by 2.6-11.8 and 26.9-36.9% at the two different growth periods, respectively. These results suggested that enhanced C / N ratio by high temperature and nitrogen application may result in the reduction of insectiocidal efficacy in Bt cotton, especially in peak boll period.