Methylamine radicals (CH3NH) and amino radicals (NH2) are major products in the early pyrolysis/ignition of monomethylhydrazine (CH3NHNH2). Ab initio kinetics of thermal decomposition of CH3NH radicals was analyzed by RRKM master equation simulations. It was found that beta-scission of the methyl H-atom from CH3NH radicals is predominant and fast enough to induce subsequent H-abstraction reactions in CH3NHNH2 to trigger ignition. Consequently, the kinetics of H-abstraction reactions from CH3NHNH2 by H-atoms was further investigated. It was found that the energy barriers for abstraction of the central amine H-atom, two terminal amine H-atoms, and methyl H-atoms are 4.16, 2.95, 5.98, and 8.50 kcal mol(-1), respectively. In units of cm(3) molecule(-1) s(-1), the corresponding rate coefficients were found to be k(8) = 9.63 x 10(-20)T(2.596) exp(-154.2/T), k(9) = 2.04 x 10(-18)T(2.154) exp(104.1/T), k(10) = 1.13 x 10(-20)T(2.866) exp(-416.3/T), and k(11) = 2.41 x 10(-23)T(3.650) exp(-870.5/T), respectively, in the 290-2500 K temperature range. The results reveal that abstraction of the terminal amine H-atom to form trans-CH3NHNH radicals is the dominant channel among the different abstraction channels. At 298 K, the total theoretical H-abstraction rate coefficient, calculated with no adjustable parameters, is 8.16 x 10(-13) cm(3) molecule(-1) s(-1), which is in excellent agreement with Vaghjiani's experimental observation of (7.60 +/- 1.14) x 10(-13) cm(3) molecule(-1) s(-1) (J. Phys. Chem. A 1997, 101, 4167-4171, DOI: 10.1021/jp964044z).