The dispersion interaction plays an important role in the organic-metal interaction, particularly in the case where the organic molecule is weakly bound to the metal substrate. Here, we report the effects of the dispersion interaction on the equilibrium structure of adsorbed methanol and the energetics of methanol decomposition via the C-H and O-H bond scissions on Pt(111) based on the results of density functional theory calculations by PBE and optB86b-vdW functionals. The binding mechanism of methanol with the surface is also clarified by the detailed analysis of the density of states, the electron density difference, and the effective Bader charge. The DFT calculated results show that the dispersion interaction significantly reduces the equilibrium distance and increases the adsorption energy of methanol on the top site of Pt(111) to 0.63 eV, thereby making it accord with the experimental result by single-crystal adsorption calorimetry. Moreover, the DFT calculations by PBE functional predict methanol tends to desorb from Pt(111) rather than undergoes decomposition whereas those by optB86b-vdW functional demonstrate that the dispersion interaction facilitates methanol decomposition via the C-H bond scission. Our theoretical results provide new insights into the surface chemistry of methanol on Pt (111) under ultrahigh vacuum conditions.