Abstract: Relative fitness of lambda cyhalothrin resistant(R), susceptible(S) and the F 1 hybrids(RS), (SR),control(CK) strains of Spodoptera exigua were studied in the laboratory. Egg production, egg hatchability, developmental period of larva and pupa, and weight of pupa, etc. were investigated in each strain. The results showed that there were significantly biological disadvantage in R strain and CK strain (the came source as R strain without indoor insecticide selection). The intrinsic rates ( R 0) of natural increase in the R, S, RS, SR and CK strains calculated by establishing life table were 117.8, 624.7, 589.1, 642.3, 218.8, respectively. Rate of relative fitness of R strain and CK strain were 0.188 and 0.368 based on comparing with that of S strain, but the value are 0.934 and 1.028 in RS and SR strain.