The atmospheric degradation mechanism of dicarboxylic acids (DCAs) initiated by hydroxyl radicals has been theoretically investigated at the DLPNO-CCSD(T)/def2-TZVP//BH&HLYP/6-311++G(d,p) level of theory. In the presence of O2, the degradation of DCAs by hydroxyl radicals takes place through a two-step mechanism: the α-H elimination and the degradation of the peroxyl radical intermediate. The latter degradation mechanism is easy to proceed for the exothermic process of radical recombination. Therefore, the degradation rate of DCAs is determined by an α-H elimination step, which is accelerated in the case of long carbon-chain DCAs with a lower energy barrier. Canonical variational transition state theory has been employed to estimate the rate constants of the H-elimination step of the DCA degradation reaction by hydroxyl radicals over the temperature range of 220-1000 K.