Abstract: Synergism of PBO and SV 1 and 3 major components(cytochrome P450, cytochrome b 5, cytochrome c reductase) of mixed-function oxidases (MFO) and its tissue distribution were compared between a fenvalerate-resistant strain (KQR) and a susceptible strain (HDS) of Helicoverpa armigera. Both PBO and SV 1 significantly increased the toxicity of fenvalerate to KQR strain, the synergistic ratio(SR) was 253.99- and 71.44- fold respectively, suggesting the important role of MFO in the resistance of KQR strain of -H. armigera.- The total cytochrome P450, cytochrome b 5, NADPH-cytochrome c reductase of KQR strain were 1.76-, 1.57-, 1.40-fold higher than HDS strain in midgut microsomes, 3.12-, 6.28-,1.88-fold higher in fat body microsomes and 4.15-, 2.39-, 1.64-fold higher in integument microsomes, respectively. The study also showed the tissue distribution patterns of the three components.