Western flower thrips (WFT), Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is a major pest of horticultural crops all over the world. Chemical control is the main strategy applied in order to control WFT. However, it is difficult to achieve because of several factors among which short generation time, high fecundity, thigmokinetic behavior, haplodiploid breeding system and development of insecticide resistance, can be mentioned. Hitherward, insecticide resistance has been reported for several insecticide groups including: organochlorines, organophosphates, carbamates, pyrethroids and spinosyns. In this research, insecticide resistance of western flower thrips was investigated in Iran. Accordingly, the resistance of five Iranian strains of WFT collected from Varamin (Tehran), Aaveh (Markazi), Nazar Abad (Alborz), Alborz Industrial Park (Qazvin) and Isfahan were examined to dichlorvos (one recommended insecticide for control the thrips in Iran). Based on the results, LC50 values were obtained 369.979 (317.317-415.658), 598.594 (514.784-667.282), 673.399 (605.578-721.573), 433.237 (359.421-486.681) and 334.104 (283.493-389.337), respectively. The most sensitive and resistant strains were from Isfahan and Alborz. Also, resistance ratio were obtained 1.10, 1.79, 2.01 and 1.29 fold, respectively. The synergistic effects of DEM, PBO and TPP in the most resistant strain showed high esterase activity with a synergistic ratio equal to 1.30 fold. Base on the results, WFT is resistant to dichlorvos and it is necessary to assay role of detoxification enzymes conferring in resistance. Further experiments are in progress. This is the first report on the resistance of western flower thrips to insecticides in Iran. So it is recommended that dichlorvos will be removed from the chemical control program of this pest or used alternatively with effective insecticides because the most common insecticide resistance management tactic is rotation between different groups of insecticides.