Controlling the particle size in the sample preparation process improves the uniformity and representativeness of the sample, enabling more accurate results on analysis. The effects of three types of mills (M1, M2 and M3) were tested in samples preparation of quinoa grains for method validation of seven pesticides (chlorothalonil, heptachlor, captan, α-endosulfan, dieldrin, β-endosulfan and endosulfan sulfate), using QuEChERS and GC-ECD. The method achieved a good chromatographic separation for the seven compounds in 12 minutes. The QuEChERS method was adjusted by changing the type of solvent used for the extraction process to a mixture of ethyl acetate with 1% glacial acetic acid. According to the results, the sensitivity of QuEChERS was influenced by particle size, showing an increase of matrix effects for four of the seven compounds studied and the particle size of 10 μm using cryogenic grinding improved the recovery percentages of method.