Bemisia tabaci is one of the major pests in the world, causing severe economic damage in more than 60 agricultural, ornamental, and commercial plants and as a sucking pest, greatly reduces crop yield. The necessity of chemical control is inevitable with regard to the high amount of economic damage and polyphagous nature of the insect. The repeated application of insecticides leads to resistance of the pest. To evaluate B.tabaci resistance to organophosphorus insecticides (dichlorvos and chlorpyrifus) , five pest populations were collected from Isfahan (Oshtorjan, Shahre abrisham 1 ,Shahre Abrisham 2, Isfahan University of Technology and Kashan), and three others from Jiroft, Gorgan and minab. The mean lethal concentrations (LC 50 ) of different populations against dichlorvos were estimated from 30.7 to 557.69 mg a.i/L which showed the difference of B.tabaci sensitivity to this insecticide. The population of Shahre abrisham 1 with the lowest mean of LC 50 value was considered as a sensitive population and Minab population with a resistance ratio of 18.163 fold, showed the highest resistance level in comparison to other populations. Against chlorpyrifos, most populations of B.tabaci except Kashan showed some resistance levels. Resistance ratio against to chlorpyrifos was between 5.1 to 32.5 fold. Resistance against dichlorvos and chlorpyrifos in Minab and Jiroft populations possibly shows the cross resistance beween organophosphorus insecticides. The influence of pre-treatment with piperonyl butoxide synergists (PBO), triphenylphosphate (TPP), and diethyl maleate (DEM) was used to determine the role of detoxification enzymes of cytochrome P450 monoxygenases, carboxylesterases and glutathione S-transferases in resistance development, respectively. Based on the results, the role of general esterases and glutathion S-transferases showed aminor role in the resistance of the tested populations of B.tabaci to dichlorvos and chlorpyrifus. However The citochorome P450 showed a significant effect in the resistance population of Jiroft. The activity of acetylcholynesterase enzyme (target site of organophosphorous insecticides) and mean inhibitory cancentration (IC 50 ) of insecticides, which inhibits 50 percent of the enzyme activity, were determined in different populations in order to evaluate insensitivity of the target site in development of resistance to organophosphorous insecticides. The results showed that enzyme activity in sensitive population was significantly lower than two resistance populations and this population also showed a lower IC 50 against dichlorvos. According to the results, it can be said that the level of activity of the acetylcholinesterase enzyme, the reible enzyme for the formation of resistance to dichlorvos and chlorpyrifos insecticides in B.tabaci populations, is in this study . Keywords: Metabolic Resistance, Target Site sensitivity, Acetylcholinesterase enzyme activity, Organophosphates