Abstract: Taking Bt cotton Sikang 1 (a conventional cultivar), Sikang 3 (a hybrid cultivar) from China and 99B (a conventional cultivar), Daiza 1 (a hybrid cultivar) from USA as test materials, the effects of different high temperatures on the square Bt proteins expression and nitrogen metabolic physiology were investigated. The results showed that the square Bt protein contents of the four cultivars decreased significantly above 38 ℃ compared with that at 32 ℃. The higher the temperature was above 38 ℃, the greater the reduction extent of the Bt protein content was. The square Bt protein contents of the hybrid cultivars were higher than that of the conventional cultivars, and were less reduced under the high temperature stress. The cultivars with bigger reductions in Bt protein content also showed greater reductions in the square soluble protein contents, pyruvic transaminase activities and glutamic oxaloacetic transaminase activities, while larger increments were detected for the square free amino acid contents, proteinsase activities and peptidase activities.