Abstract: Toxicology of PBO on third instar cotton budworm (Helicoverpa armigera) larva has been studied in this paper. The results show that, LD 50 of PBO on third instar cotton budworm is 5.7532μg/larva. With the treatment of PBO at 4μg/larva for 1 hour, AChE, CarbE,MFO are all inhibited, and the inhibition rates are 48.31%, 44.97% and 60.82%, and I 50 s are 3.35×10 -7 mol/L, 4.46×10 -7 mol/L and 1.42×10 -7 mol/L respectively. Moreover, the inhibited AChE and CarbE are unreversible. All these prove that PBO has the capacity to inhibit more than one enzyme and has certain toxicity to cotton bollworm just like an organophosphorous pesticide. So we can not justify whether MFO is involved in resistance development only according to the synergistic result of PBO. In addition, specific effects of TPP and DEM have also been confirmed.