Three possible pathways for C–N bond breaking in methylamine have been investigated over clean Mo(100) and nitrogen atom-modified Mo(100) surfaces with a nitrogen coverage of 0.25 monolayer (ML) (N–Mo(100)) firstly, and the C–N bond breaking following the intramolecular hydrogen transfer from the CH₃ to NH₂ is excluded owing to the high barriers. Then methylamine decomposition starting with C–H, N–H, and C–N scission over the nitrogen atom-modified Mo(100) surface with a nitrogen coverage of 0.5 ML (2N–Mo(100)) has been systematically investigated, and the decomposition network has been mapped out. The thermochemistry and energy barriers for all the elementary steps, starting with C–H, N–H, and C–N scission, and sequential reactions from the resulting intermediates, are presented here. The most likely decomposition path is H₃CNH₂ → H₂CNH₂ + H → HCNH₂ + H + H → CNH₂ + H + H + H → C + NH₂ + H + H + H → C + NH₃ + H₂ → C + NH₃(g) + H₂(g). For the decomposition reactions involved in the likely decomposition path, there is a linear relationship between the energy of transition state and the energy of final state. For the reverse processes of the dehydrogenation of CH, NH, NH₂, it is found that there is a linear relationship between the barrier and the valency of A (AC, N, and NH).