The kinetics of H-abstraction reactions of monomethylhydrazine (MMH) by OH radical was investigated using ab initio transition state theory with correction of quantum mechanical tunneling effects. The stationary points of the potential energy surfaces were calculated at the QCISD(T)/CBS level. For the H-abstraction of primary and secondary amine H atoms by OH radical, the geometries were optimized using the multireference theory at the CASPT2/aug-cc-pVDZ level due to the multireference character, and the energies were found to be 1.5 and 3.5 kcal mol-1 (0 K) lower than that of the entrance channel. The geometry for the H-abstraction of methyl hydrogen at the B3LYP/6-311++G(d,p) level was found to have a six-center transition state with the energy barrier of 1.2 kcal mol-1 (0 K). By the intrinsic reaction coordinate (IRC) analysis, two hydrogen-bonded complexes with energies of 6.5-6.9 kcal mol-1 (0 K) lower than that of the entrance channel were found to connect the reactants and the three transition states for the H-abstraction of amine and methyl H atoms, and three MMH radical-H2O complexes were found to connect three product channels and the corresponding transition states. Consequently, the rate coefficients of MMH + OH were determined based on the two-transition-states model, with E and J conserved between the transition-state regions. The calculated rate coefficient of the H-abstractions of MMH + OH is in good agreement with available experimental data.