Abstract: Potential mechanism of resistance to abamectin was examined in susceptible strain (SS) and resistant strain (RS) of diamondback moth, Plutella xylostella L. The levels of cytochrome P450, cytochrome b5, NADPH-cytochrome P450 reductase and four indices of monooxygenase activities methoxyresorufin-O-demethylase (MROD), ethoxyresorufin-O-deethylase (EROD), ethoxycoumarin-O-deethylase (ECOD), and p-nitroanisole-O-demethylase (PNOD) were compared between the SS and the RS. During the different developmental stages, the resistant strain showed significantly elevated levels of cytochrome P450 than that in the susceptible strain,except adult stages. In comparison to SS, RS had higher activities of P450 reductase, MROD, EROD, ECOD and PNOD, the ratio were 1.97-, 9.41-, 4.15-, 1.67- and 2.94-fold, respectively. These results suggested that the cytochrome P450 may play a role in the insecticide resistance. Abamectin resistance was correlated with cytochrome P450 monooxygenases in diamondback moth.