The gas-phase kinetics of H-abstraction reactions of monomethylhydrazine (MMH) by OH radical was investigated by second-order multireference perturbation theory and two-transition-state kinetic model. It was found that the abstractions of the central and terminal amine H atoms by the OH radical proceed through the formation of two hydrogen bonded preactivated complexes with energies of 6.16 and 5.90 kcal mol(-1) lower than that of the reactants, whereas the abstraction of methyl H atom is direct. Due to the multireference characters of the transition states, the geometries and ro-vibrational frequencies of the reactant, transition states, reactant complexes, and product complexes were optimized by the multireference CASPT2/aug-cc-pVTZ method, and the energies of the stationary points of the potential energy surface were refined at the QCISD(T)/CBS level via extrapolation of the QCISD(T)/cc-pVTZ and QCISD(T)/cc-pVQZ energies. It was found that the abstraction reactions of the central and two terminal amine H atoms of MMH have the submerged energy barriers with energies of 2.95, 2.12, and 1.24 kcal mol(-1) lower than that that of the reactants respectively, and the abstraction of methyl H atom has a real energy barrier of 3.09 kcal mol(-1). Furthermore, four MMH radical-H2O complexes were found to connect with product channels and the corresponding transition states. Consequently, the rate coefficients of MMH + OH for the H-abstraction of the amine H atoms were determined on the basis of a two-transition-state model, with the total energy E and angular momentum J conserved between the two transition-state regions. In units of cm(3) molecule(-1) s(-1), the rate coefficient was found to be k(1) = 3.37 x 10(-16)T(1.295) exp(1126.17/T) for the abstraction of the central amine H to form the (CH3NNH2)-N-center dot radical, k(2) = 2.34 X 10(-17)T(1.907) exp(1052.26/T) for the abstraction of the terminal amine H to form the trans-(CH3NHNH)-H-center dot radical, k(3) = 7.41 X 10(-20)T(2.428) exp(1343.20/T) for the abstraction of the terminal amine H to form the cis-(CH3NHNH)-H-center dot radical, and k(4) = 9.13 x 10(-21)T(2.964) exp(-114.09/T) for the abstraction of the methyl H atom to form the (CH2NHNH2)-H-center dot radical, respectively. Assuming that the rate coefficients are additive, the total rate coefficient of these theoretical predictions quantitatively agrees with the measured rate constant at temperatures of 200-650 K, with no adjustable parameters.