The sublethal effects of the insecticide spinosad were assessed against Plutella xylostella (L.) (Lepidoptera: Plutellidae) by using a demographic approach. The resulting data were analyzed based on the age-stage and two-sex life table model. Bioassay results showed that the susceptibility of P. xylostella to spinosad decreased in Sub strains selected by LC₂₅ spinosad for 10 generations. Although the egg size was significantly smaller in Sub-1 (selected by LC25 spinosad for one generation) compared with spinosad-susceptible strain SS, egg size recovered to normal levels in Sub-5 (selected by LC₂₅ spinosad for five generations) and were even larger in Sub-10 (selected by LC₂₅ spinosad for 10 generations). The life-history parameters changed in the Sub strains of different generations. Sublethal spinosad had a significant effect on Sub-1; however, the sublethal effect on the Sub strains decreased as selection cycles increased. The intrinsic rate of increase (r_m), finite rate of increase (λ) and net reproductive rate (R₀) in Sub-1 were significantly decreased compared with those in the SS strain. No significant differences were found between the Sub-5 or Sub-10 compared with the SS strain. The greatest difference was observed in the total number of eggs laid by each female, i.e., the fecundity. The fecundity in SS, Sub-1, Sub-5, and Sub-10 were 121.92, 81.26,114.42, and 159.21, respectively. The life expectancy of an SS, Sub-1, Sub-5, and Sub-10 egg was 10.58, 8.71, 10.21, and 7.45 d, respectively.