Inelastic collision processes of CH4 at a Pt(1 1 1) surface have been analyzed by means of super-sonic molecular beam technique. Obtained results of the angular intensity distribution of the reflected CH4 with the incident kinetic energy ranging from 190 to 350 meV are found to qualitatively agree with the prediction from the classical collision model. Surface temperature dependence of the speed distributions of the reflected CH4 qualitatively follows the prediction at the surface temperature below 500 K, but deviates from the prediction as the surface temperature increases under the irradiation of CH4 with higher kinetic energies, where decomposition of CH4 takes place. The discrepancy of data from the model can be understood by introducing into the model decreased effective surface mass and increased surface " roughness".