Cytochrome P450 monooxygenases of insects play an important role in the detoxification of both phytotoxin and chemical insecticide,which are considered to be related to the resistance of insects. In this study,a P450 monooxygenase gene CYP9A14 of cotton bollworm Helicoverpa armigera (Hübner),was found to be 17-fold overexpressed in a cypermethrin resistant strain compared to the sensitive strain by real-time PCR. A 490 bp CYP9A14 gene from H.armigera midgut was cloned and used to construct a recombined helicoverpa nuclear polyhedrosis virus (HaNPV)) by a double-stranded RNA interference (dsRNAi) manner. When H.armigera cypermethrin resistant larvae were injected with the recombined HaNPV,level of the CYP9A14 transcript in the midgut significantly decreased in comparison with the control construction.