To provide insight on the reaction mechanism of the methyperoxy (CH3O2 center dot) self-reaction, stationary points on both the spin-singlet and the spin-triplet potential energy surfaces of 2(CH3O2 center dot) have been searched at the B3LYP/6-311++G(2df,2p) level. The relative energies, enthalpies, and free energies of these stationary points are calculated using CCSD(T)/cc-pVTZ. Our theoretical results indicate that reactions on a spin-triplet potential energy surface are kinetically unfavorable due to high free energy barriers, while they are more complicated on the spin-singlet surface. CH3OOCH3 + O-2(1) can be produced directly from 2(CH3O2 center dot), while in other channels, three spin-singlet chain-structure intermediates are first formed and subsequently dissociated to produce different products. Besides the dominant channels producing 2CH(3)O center dot +O-2 and CH3OH + CH2O + O-2 as determined before, the channels leading to CH3OOOH + CH2O and CH3O center dot + CH2O + HO2 center dot are also energetically favorable in the self-reaction of CH3O2 center dot epecially at low temperature according to our results.