Abstarct: Rapeseed is one of the most important oilseed crops that plays a major role in providing human edible oil. Cabbage aphid, Brevicoryne brassicae L. , is one of the most important and serious pests of rapeseed in Iran. In most areas, farmers spray insecticides several times to control it. If the pest is resistant to the insecticides used, not only the pest is not controlled but it also increases production costs and environmental problems. Therefore, this study was conducted to investigate the susceptibility of cabbage aphid populations in Semnan province to common insecticides used against it. In this study, the effect of different concentrations of oxidimethonomethyl, imidacloprid, primicarb and combined treatment of oxidimethomethyl, imidacloprid on three populations of this aphid from Semnan, Shahroud and Garmsar in the crop year 2020 in the laboratory conditions. Each of the insecticides was used in five concentrations , recommended concentration, two higher and two lower concentrations of recommended concentration including primicarb (500, 750, 1000, 1250 and 1500 ppm), imidacloprid (250, 375, 500, 625 and 750 ppm), oxydimethone methyl (500, 750, 1000, 1250 and 1500 ppm) and combination treatment (375, 562. 5, 750, 937. 5, 1125 ppm) were used. Biometric tests were performed by leaf dip method and aphid mortality was recorded 24, 48, 72 and 96 hours after treatment with insecticides. At the recommended dose of the tested insecticides, the highest mortality with 86. 25% was related to oxy dimethyl methyl-imidaclopride in Shahroud population and the lowest mortality with 62. 5% was related to oxy dimethyl methyl on Garmsar and Shahroud populations. The amount of LC50 obtained for oxy dimethyl methyl in Garmsar, Semnan and Shahroud populations was equal to 264. 77, 268. 07, 201. 53 respectively, for the combined treatment of oxy dimethyl methyl and imidacloprid 163. 82, 109. 98, 48. 99 for imidacloprid 169. 87, 155. 27, 177. 22 and for primicarb treatment 152. 68, 158. 57 and 89. 86 respectively. According to these results, it can be said that the population of Shahroud, except for imidacloprid treatment, responded better to other treatments and was recognized as a sensitive population. The LT50 obtained for the toxins used showed that the combined treatment of imidacloprid-oxydimethonomethyl with 38. 7 hours was the lowest and imidacloprid with 54. 5 hours was the highest LT50. The results of this study can be effective in developing an integrated pest management plan for Brevicoryne brassicae L.