Abstract: Relative fitness of the R, the homologous control population(S), and the F1 hybrids(RS) were tested.The results showed that there were significantly fewer eggs per female, lower egg hatchability and pupation rate, and significantly longer duration of larva and pupa in R population compared with that in S population.Relative fitness of R population is only 0.65 based on comparing R0 of R population with R0 of S population, but the value is 1.6 in RS population. The results indicated that there are some reproductive disadvantages in R population. No significant difference of toxicity of Cry1Ac to the two reciprocal cross populations between R and S populations was observed. This indicated that the resistance to Bt ICP was inherited as antosomal factor.