Abstract: A methoxyfenozide-resistant population and a homologous susceptible population of Helicoverpa armigera(Hübner) were selected in the laboratory.The resistant population showed 72.61-fold resistance to methoxyfenozide compared with susceptible one.Resistant mechanisms of H.armigera to methoxyfenozide were preliminary studied by synergism assay using piperonyl butoxide,phosphate defoliant and diethyl maleate in vivo,and by enzyme activity assay for mixed function oxidases,general esterases,and glutathione-S-transferases in vitro.The results showed that enhancement of activity of the three enzymes may contribute to methoxyfenozide resistance in the resistant population of H.armigera.