Systematic investigations were carried out on the effect of Bt transgenic cotton on the population dynamics of target pest, cotton bollworm, Helicoverpa armigera (Hubner) in Nha Ho (Ninh Thuan) during dry season. This experiment included three treatments. The first was a Bt transgenic cotton field (female variety: CS95), the second was a Bt hybrid cotton field (VN15), and the third was a non-Bt cotton field (male variety: LRA5166). Results showed that Bt-gene does not influence on egg-laying of cotton bollworm, but exuberant plant of VN15 is attractive to cotton bollworm females. Bollworm eggs were laid at the same time on Bt cotton (CS95, VN15) and non-Bt cotton (LRA5166). The population trends of bollworm eggs were similar in all treated cotton fields. But, the cumulative numbers and peak of abundance of bollworm eggs in the VN15 field (Bt hybrid cotton) were obviously always higher than that in the LRA5166 field (non-Bt cotton). The population trends of bollworm larvae were very different from each other in all treated cotton fields. The time of initial appearance of larvae in non-Bt cotton field (LRA5166) was recorded at 32 days after emerging. Conversely, larvae appeared very late in Bt transgenic cotton field and Bt hybrid cotton field, only at 67 and 53 days after emerging, respectively. The peaks of abundance of larvae occurred at various times in all treated cotton fields. The densities and peak of abundance of bollworm larvae in the non-Bt cotton field (LRA5166) were obviously higher than that in the Bt cotton fields (CS95 and VN15). That means the population trends of bollworm larvae in the fields of CS95 and VN15 were significantly affected by Bt-gene.