The difference for resistant effects to bollworm, the dynamic of resistance and the content of Bt toxin protein in different organs during developmental stages of different varieties of Bt transgenic cotton were explored in laboratory. The results showed that there were significant differences for resistance to cotton bollworm during every growing stage of all varieties; DP 99B has the highest resistance to cotton bollworm, while Hanza 154 has the lowest, and their average corrected mortality to cotton bollworm were 89.23% and 75.91%, respectively. The resistance to cotton bollworm of cotton plants in each growing stage was presented in the following order, budding stage &gt; flowering stage &gt; flowering-boll stage &gt;boll stage. The results of ELISA showed that Bt toxin protein could be detected in all organs of transgenic cotton, but its content varied significantly among organs along the developmental stages. There were significant differences of Bt toxin protein content among the functional leaves in different stages, the order of the content was: seven-leave stage &gt; three-leave stage &gt; budding stage &gt; boll stage &gt; flowering-boll stage.