The mechanism of formate decomposition on a NiO(111) surface is investigated using the density functional method and (NiO)(x) cluster models. The reaction consists of three steps: (1) configuration change from bidentate to monodentate, (2) HCOO rotation around the CO axis, and (3) H atom transfer from the formate to the surface O atom. The bidentate form is more stable by 21 kJ/mol than the monodentate one, and the energy barrier measured from the former is 25 kJ/mol, which is consistent with the recent experimental finding that the mutual conversion is an equilibrium reaction. There is no essential reaction barrier during the HCOO rotation about the CO axis. The migration of H atoms from formate to the surface O atom is the rate-limiting step, and the activation energy is estimated to be 73 kJ/mol. The product is the surface OH groups and adsorbed CO2 molecule, which is consistent with the recent experimental stidies.