Abstract: Methomyl, carbaryl, phoxim,parathion, dichlorvos and omethoate were mixed each other by the ratio of 1∶1, 1∶3 and 3∶1. Inhibited median concentrations (I50) of these insecticides and their mixtures on acetylcholinesterase (AChE) and carboxylesterase (CarE) in cotton bollworm, Helicoverpa armigera, were tested. The synergism of the mixtures were analyzed by Joint-inhibition coefficient. It was concluded that the JIC of the mixtures to AChE were different from that to CarE, and the JIC of these mixtures varied greatly according to the proportion of the mixture. Parathion +carbaryl(1∶3), phoxim +parathion (1∶3) and methomyl + omethoate(1∶3) were synergistic to AChE and CarE, while carbaryl +dichlorvos(1∶3),carbaryl +omethoate(1∶1),phoxim +carbaryl(1∶1), parathion +omethoate(1∶1), parathion +omethoate(1∶3), dichlorvos + omethoate(1∶3)and methomyl + parathion(1∶1) were antagonistic to AChE and CarE.