Oxidation mechanism of Pyrene (Py) by OH radicals is studied in two possible reactions by density functional theory (DFT) at the M06-2X/6-311++G(2df,2p) theoretical level. OH-addition reaction in compare with the H-abstraction reaction is a more favored pathway in the gas-phase. Hence, the studies continued for investigating the kinetic aspects of the OH-addition reaction. Furthermore, Rice-Ramsperger-Kassel-Marcus (RRKM) theory as well to calculate the kinetic rate constant for the initial steps and investigate the effects of diversity in pressure and temperature. Pathway 2 (attack onto C2 carbon atom) and following that forming of the related complex which has a lower energy barrier is the most efficient process. The good agreement with the available experimental data reveals that a two-step reaction scheme prevails. These ratios also show a decrease in the regioselectivity by increasing the temperatures and decreasing the pressures. The Pyrene lifetime in the atmospheric condition with OH is estimated ∼5.5 days. An atom in Molecules theory (AIM) in critical point and natural bond orbital (NBO) analysis use to describe the nature of covalent interaction, and is as a confirmation of DFT. The NBO analysis shows a decrease in the HOMO-LUMO gaps to form the stable complex, which is parallel with the decreasing of activation energy.