An insecticide resistance detection bioassay on sweet potato whitefly, Bemisia tabaci (Hompotera: Aleyrodidae), a major Omani vegetable insect-pests was carried out under laboratory conditions for acetyl cholinesterase inhibition. The whitefly samples were collected form eighteen vegetable production farms from the Northern Oman (AlBatinah region). Technical grade malathion, diazinon, dimethoate and chloropyrifos with different concentrations (0, 5, 10, 25, 50 and 100 ppm) were tested for their insect enzyme inhibition activity through Microplate reader system. In-vitro enzyme assay data showed different levels of insecticide resistance in whitefly populations collected from different fields. For instance, the lowest inhibition rate (IR) for diazinon was recorded 35.08 % and the highest value was 70.00 % by 30 minutes and at concentration of 100 ppm. For malathion IR value range from (72.00 to 87.00 %) by 30 minutes and at concentration of 100 ppm. Dimethoate IR value ranged from 66.67 to 85.05% by 30 minutes and at concentration of 100 ppm. For, chlorpyrifos the maximum R values of 59.50% and the lowest value of 46.00% by 30 minutes and at chlorpyrifos concentration of 100 ppm. Despite their different fields locations, insect samples, showed variable inhibition rates toward the four insecticides, as 87.00 % to malathion, 85.05% to dimethoate followed by diazinon (70.00 %) and the lowest to chlorpyrifos (59.50%) by 30 minutes and at concentration of 100 ppm. When same whitefly samples (collected from same field) were compared for their resistant level to two different insecticides (diazinon and malathion), variations in insecticides resistance were observed, When IR (median inhibition rate) were compared between diazinon and malathion, results showed that malathion was more toxic (IR50= 16.94 ppm) than diazinon (IR50= 144.03 ppm). Whitefly population showed high level of resistance, i.e. 93.42-12.10 fold to diazinon, 6.88-11.85 folds to Malathion, 13.97-17.10 folds to dimethoate and 592.33-22.67 fold to Chlorpyrifos. Current results indicate that the whitefly (B.tabaci) populations in different vegetables farms in Al-Batinah region had different degree of resistance against organophosphorus insecticides, especially Malathion, Diazinon, Dimethoate and Chlorpyrifos.