Laboratory and field studies were conducted to compare the insecticidal activity of four transgenic Bt cotton lines,GK 12,GK 2,R108,developed in China and NuCOTN 33 B developed by Monsanto Company against Helicoverpa armigera. Among the four Bt cotton lines, GK 12 and NuCOTN 33 B showed much higher insecticidal activity against H. armigera both in laboratory and field studies. There were no significant differences in insecticidal activity between the two cotton lines.