The small brown planthopper (Laodelphax striatellus (Fallén), Hemiptera: Delphacidae), is an important agricultural pest of rice, and neonicotinoid insecticides are commonly used for controlling L. striatellus. However, the sublethal effects of thiamethoxam on L. striatellus remain relatively unknown. In this study, an age-stage life table procedure was used to evaluate the sublethal effects of thiamethoxam on the biological parameters of L. striatellus. Additionally, activities of carboxylesterase, glutathione S-transferase, and cytochrome P450 monooxygenase in the third instar nymphs were analyzed. The results indicated that the survival time of F₀ adults and the fecundity of female adults decreased significantly after the third instar nymphs were treated with sublethal concentrations of thiamethoxam (LC₁₅ 0.428 mg/liter and LC₃₀ 0.820 mg/liter). The developmental duration, adult preoviposition period, total preoviposition period, and mean generation time of the F₁ generation increased significantly, whereas the fecundity of the female adults, intrinsic rate of increase (r_i), and finite rate of increase (λ) decreased significantly. The oviposition period was significantly shorter for the insects treated with LC₃₀ than for the control insects. Neither sublethal concentrations had significant effects on the adult longevity, net reproduction rate (R₀), or gross reproduction rate (GRR) of the F₁ generation. The activities of carboxylesterase, glutathione-S-transferase, and cytochrome P450 monooxygenase increased significantly after the thiamethoxam treatments. These results indicate that sublethal concentrations of thiamethoxam can inhibit L. striatellus population growth and enhance detoxification enzyme activities.