The H-abstraction reactions of MMH by the OH radical generate three radical intermediates: CH3N·NH2, CH3NHN·H, and C·H2NHNH2, which undergo further decomposition to smaller radicals and stable products. It was found that the β-scission of H atom from the CH3N·NH2 radical is not feasible as those of the β-scission of NH2 and CH3 from the C·H2NHNH2 and CH3NHN·H radicals due to its higher energy barrier. Consequently, it may react with the OH radical for further oxidation and decomposition. The reactions of CH3N·NH2 with the OH radical was studied in details using ab initio and DFT methods. The geometries of the stationary points were optimized using multireference CASPT2(4e,3o)/aug-cc-pVDZ and density functional B3LYP/6-311++G(d,p) theories, with energies refined at the QCISD(T) level with the basis set extrapolated to the CBS limit via extrapolation of the cc-pVTZ and cc-pVQZ results. The transition state of the CH3N·NH2 + OH reaction for the formation CH3N(OH)NH2 adduct was evaluated with the variable reaction coordinate transition state theory employing the multireference CASPT2(4e,3o)/aug-cc-pV∞Z interaction energies. It was found that the CH3N(OH)NH2 adduct undergoes direct decomposition via a four-center transition states to CH2=NNH2 + H2O products, it also can isomerize to CH3NNH2OH and then further decompose to CH3N=NH+H2O via a small energy barrier. The results reveal that the H-abstractions of the primary amine hydrogen by the OH radical via a hydrogen bonded complex are important to methyldiazene product formation.