Abstract: The relative biological fitness of Plutella xylostella L. was evaluated by constructing and comparing the life tables of three strains resistant to abamectin(A), beta-cypermethrin(B), thiosultap-disodium(D) respectively, the susceptible strain(S) and six F1 hybrid progenies from the reciprocal hybrids of the three resistant strains. The results revealed that the relative fitness values of the three resistant strains were 0.60, 0.77 and 0.53, respectively, which were lower than that of the susceptible strain (relative fitness value as 1.0); and the relative fitness values of six F1 hybrid strains (A♀+B♂, A♂+B♀, A♀+D♂, A♂+D♀, B♂+D♀ and B♀+D♂) were 0.67, 0.53, 0.67, 0.74, 0.73 and 0.80, respectively, which were also lower than that of the susceptible strain, but were similar to or higher than that of the three resistant strains.