Abstract: Relative fitness of cyhalothrin-resistant(R), -susceptible (S) and the F1 hybrids(RS) strains of Helicoverpa armigera Hübner was evaluated in a field cage trial in 1997 in Beijing, P. R. China. Egg production, egg hatchability, developmental period of larva or pupa, and weight of pupa were investigated in each strain. The results showed that there are significantly fewer egg production per female, lower egg hatchability and longer period of larval development in R strain compared with that in S strain. The intrinsic rates (R0) of natural increase in the R, S, and RS strains calculated by establishing life table are 15.17, 38.13, and 55.79,respectively. Relative fitness of R strain is only 0.46 based on comparing R0 of R strain with that of S strain, but the value is 1.46 in RS strain.