Onion thrips, Thrips tabaci is one of the most important pests of onion, garlic and several vegetable crops. Its control relies mainly on frequent spraying that result in quick resistance. Several pesticides from different chemical groups are used against this pest. In this research, onion thrips resistance to four different insecticides was evaluated at 10 onion-growing regions of Isfahan including Harand, Shahre-Abrisham, Borkan, Dorcheh, Ghahdarijan, Heydar-Abad, Daneshgah, Jar, Zazeran and Ziar. The main mechanisms of resistance and possibility of cross resistance were also examined. The pesticides were profenofos and chlorpyrifos-ethyl (organophosphates), imidacloprid (neonicotinoids) and deltamethrin (pyrethroids). Since the population collected from Harand had never been exposed to pesticide spraying, we considered this population as a susceptible reference strain. LC 50 values for this reference strain were 0.63 ppm for profenofos , 13.49 ppm for chlorpyrifos-ethyl, 3.04 ppm for imidacloprid and 0.007 ppm for deltamethrin. Based on the results, all populations showed a range of resistance compared to Harand population. The highest LC 50 value against profenofos was observed in Ziar population (LC 50 =19.49). The resistance against chlorpyrifos-ethyl was low and the highest resistance ratio against this pesticide was 5.67 in Borkan population. Ghahdarijan population showed the highest resistance against imidacloprid in which resistance ratio of 12.56 was estimated. The highest resistance ratio between studied populations was against deltamethrin so that the resistance ratios in Daneshgah, Zazeran, Ziar and Heydar-Abad populations were 209.55, 116.58, 111.82 and 106.04, respectively. To investigate the role of detoxification enzymes involved in resistance, the synergistic effect of PBO (piperonyl butoxide) and DEM (diethyl maleate) in pre-treatment application was measured on the rate of resistance. The most effect of PBO as inhibitors of P 450 monoxygenases in reducing resistance was against profenofo and chlorpyrifos-ethyl so the highest synergistic ratio against profenofos and chlorpyrifos-ethyl was observed in Ghahdarijan (SR=13.33) and Borkan (SR=12.68) populations, respectively. It seems that P 450 cytochrome monoxygenases have a major role in the thrips resistance to organophosphorous pesticides. In imidacloprid toxicity, pretreatment with two mentioned synergists resulted in a synergistic ratio around one. The highest synergistic effect of PBO and DEM on the imidacloprid toxicity was observed in Ghahdarijan (SR=1.87) and Shahre-Abrisham (SR=1.10) populations, respectively, with no significant difference to Harand population. The results revealed that the examined enzymatic mechanisms have not a major role in the resistance against neonicotionoids pesticides. The results of synergists pretreatment against deltamethrin also indicated that the enzymatic mechanisms couldn’t be the main cause of resistance to pyrethroids in the tested populations. Occurrence of pyrethroid target site mutations in T. tabaci populations were also investigated using molecular assays. Thus, the DNA of Harand population as susceptible reference and Ghahdarijan and Zazeran as resistant populations were extracted and the sodium channel IIIS3-IIS5 region was amplified with specific primers. Of the reported mutations in T. tabaci sodium channel-M918T, M918L, L1014F, T929I and V1010A- we found the change from tereonin to isoleucine at position 929 in Ghahdarijan and Zazeran populations resistant to deltamethrin. In four populations with high levels of resistance, susceptibility to diazinon and dicholorvous (organophosphates), permethrin and cypermethrin (pyretroids), acetamiprid (neonicotinoids), spinosad (spinosyns) and azadirachtin (avermectins) were evaluated to determine the cross resistance spectrum. The results showed that the resistant populations to profenofos and chlorpyrifos-ethyl were also resistant to other tested phosphorous insecticides. Those populations, who had high resistance to deltamethrin, were resistant to permethrin and cypermethrin as well. Similarly imidacloprid -resistant Ghahdarijan population showed the highest resistance ratio (RR=2.36) to acetamiprid. No population was resistant to Spinosad and azadirachtin that had never been used against T. tabaci in Iran. Keywords : Thrips tabaci , resistance to pesticides, synergistic ratio, amino acid replacement, cross resistance