Thermal decomposition of CH3OH diluted in Ar has been studied by monitoring H atoms behind reflected shock waves of 100 ppm CH3OH + Ar. The total decomposition rate k(1) for CH3OH + M -> products obtained in this study is expressed as, ln(k(1)/cm(3) molecule(-1) s(-1)) = -(14.81 +/- 1.22) - (38.86 +/- 1.82) X 10(3)/T, over 1359-1644 K. The present result on k(1) is indicated to be substantially smaller than the extrapolation of the most of the previous experimental data but consistent with the published theoretical results [Faraday Discuss. 2002, 119, 191-205 and J. Phys. Chem. A 2007, 111, 3932-3950]. Oxidation of CH3OH has been studied also by monitoring H atoms behind shock waves of (0.35-100) ppm CH3OH + (100-400) ppm O-2 + Ar. For the low concentration CH3OH (below 10 ppm) + O-2 mixtures, the initial concentration of CH3OH is evaluated by comparing evolutions of H atoms in the same concentration of CH3OH with addition of 300 ppm H-2 diluted in Ar. The branching fraction for CH3OH + Ar -> (CH2)-C-1 + H2O + Ar has been quantitatively evaluated from this comparative measurements with using recent experimental result on the yield of H atoms in the reaction of (CH2)-C-1,3 + O-2 [J. Phys. Chem. A 2012, 116, 9245-9254]; i.e., the branching fraction for the above reaction is evaluated as, phi(1a) = 0.20 +/- 0.04 at T = 1880-2050 K, in the 1.3 and 3.5 ppm CH3OH + 100 ppm O-2 samples. An extended reaction mechanism for the pyrolysis and oxidation of CH3OH is constructed based on the results of the present study combined with the oxidation mechanism of natural gas [GRI-Mech 3.0]; evolution of H atoms can be predicted very well with this new reaction scheme over a wide concentration range for the pyrolysis (0.36-100 ppm CH3OH), and oxidation (0.36-100 ppm CH3OH + 100/400 ppm O-2) of methanol.