Organic peroxy radicals (RȮ2), formed by well stabilization reaction of Ṙ + O2, are important intermediates in atmospheric degradation of organic matter and low-temperature combustion of hydrocarbon fuels. A fate of RȮ2 is isomerize to QOOH , and the downstream reactions of QOOH are known to profoundly fuel reactivity. However, both experimental measurements and high-level theoretical cal-culations of RȮ2 kinetics are scarce, leading to a large gap in our understanding of RȮ2 fate. In this study, methoxy-methyl-peroxy rad-ical (CH3OCH2OȮ) is investigated as a model compound to elucidate the fate of RȮ2. Based on a high-level quantum chemical calcula-tion and RRKM/master equation calculation, we report a detailed RȮ2 unimolecular reaction surface, as well as the corresponding temperature- and pressure-dependent rate coefficients. All the calculated results can be used to develop the global modeling of SOAs in atmospheric science, to estimate the corresponding rate coefficients of larger oxygen-centered fuels, as well as to construct the chemical kinetic model of dimethyl ether in combustion science.