Cabbage aphid Brevicoryne brassicae is an important pest of crucifers in Iran and is controlled mainly with dimethoate. Some population of B. brassicae have become resistant to dimethoate due to intensive use of this insecticide. Up to now, there is no information about the mechanisms of resistance in B. brassicae population to this insecticide which can help to resistance management of this serious pest. The toxicity of dimethoate in six populations of the pest from different regions (Dezfool, Falavarjan, Kordan, Mehrshahr, Tehran and Varamin) was assayed using insect-dip method. The synergistic effects of triphenyl phosphate (TPP), piperonylbutoxide (PBO) and diethyl maleate (DEM) were evaluated on the most resistant and the susceptible population to find the involvement of esterases, mixed function oxidases and glutathione S-transferases (GSTs) in resistance mechanisms, respectively. In order to determine esterase, GSTs, and cytochrome P450 monooxygenase activities the methods of Van Asperen (1962), Habig et al. (1976) and De Sousa et al. (1995) were used respectively. The bioassay results showed significant difference in susceptibility to dimethoate among the populations. Resistance ratio of the populations to the susceptible population ranged from 2.5 to 91.25 The level of resistance to dimethoate in the resistant population was suppressed by TPP, PBO and DEM, suggesting that the resistance to dimethoate is caused by MFO and GSTs. Up to 1.04, 2.71 and 9.57-fold increase in esterase, GSTs and cytochrome P450 monooxygenases activities in resistant strains compared with the susceptible one was observed respectively. Biochemical enzyme assays showed that GSTs, esterase and cytochrome P450 monooxygenase activities in the resistant population was clearly higher than that in the susceptible one. This data help us to detect the mechanisms involved in resistance of B. brassicae to dimethoate and subsequently resistance management of this important pest.