The thermodynamics of methane decomposition on the ruthenium (1120) surface has been investigated with ab initio periodic calculations. All surface intermediates are more stable than the gas-phase methane even if the last step of the decomposition path: CH --> C + H, is highly endothermic. Among all of the surface species, CH2 appears to be the most stable. All of the surface species (CHx, x = 3-1 and H) adsorb on bridge-up sites, while atomic C prefers top-down sites. The transition states of the elementary reactions for the dissociation of methane on the ruthenium (1120) surface have been investigated with nudged elastic band method (NEB). The calculated barriers are 56 kJ mol(-1) for methane decomposition, 11 kJ mol(-1) for methyl decomposition, and 52 kJ mol(-1) for methylene decomposition, respectively. The decomposition of CHads requires the highest activation energy from the series with 95 kJ mol(-1).