Decomposition of methanthiol on Pt(111) is systematically investigated using self-consistent periodic density functional theory (DFT), and the decomposition network has been mapped out. The most stable adsorption of the involved species tends to form the sp3 hybridized configuration of both C and S atoms, in which C is almost tetrahedral and S has the tendency to bond to three atoms. Spontaneous dissociation rather than desorption is preferred for adsorbed methanthiol. Based on the harmonic transition state theory calculations, the decomposition rate constants of the thiolmethoxy and thioformaldehyde intermediates are found to be much lower than those for their formation,leading to long lifetimes of the intermediates for observation. Under the ultrahigh vacuum (UHV) condition, the most possible decomposition pathway for methanthiol on Pt(111) is found as CH3SH f CH3S f CH2S f CHS f CH t S f C t S, in which the C-S bond cleavage mainly occurs at the CHS species. However, the decomposition pathway is CH3SH f CH3S f CH3 t S under the hydrogenation condition; the C-S bond scission mainly occurs at CH3S. The Br?nsted-Evans-Polanyi relation holds for each of the S-H, C-H, and C-S bond scission reactions.