Azinphos methyl (O,O-Dimethyl S-[(4-oxo-1,2,3-benzotriazin-3(4H)-yl)methyl] phosphorodithioate) is a broad spectrum organophosphate insecticide. Like other pesticides in its class, it owes its insecticidal properties to being an acetylcholinesterase inhibitor. Azinphos methyl is often used as organophosphorus pesticides such as active Guthion, Gusathion (GUS), Gusathion-M, Crysthyron, Cotnion, Cotnion-methyl, Methyltrizotion, Carfene, Bay9027, Bay17147 and R-1852. In the study, the possible reaction pathways between the Azinphos methyl molecule and the OH radical were examined theoretically. The optimized geometries of the molecule were drawn with the Gauss-View 5 program and the lowest energy states were determined using the Gaussian 09 program. By calculating the bond lengths and bond angles of the molecules, geometric structure analysis was performed. The main purpose here is to determine the most likely produced in the reaction between Azinphos methyl and OH radical in aqueous media and gaseous media. The products formed as a result of the reactions of the azinphos methyl molecule with the OH radical were determined and their geometric optimizations were carried out by semi-empirical AM1, Ab initio Hartree Fock HF/6-31G and Functional Density Theory (DFT) methods. COSMO was used as the solving model. Thanks to this method, the area where the reaction takes place between the polarizations on the surface of the molecule and the solvent is determined. Thanks to the dielectric property of water, the reaction medium is stabilized.