The　catalytic　decomposition　of　formic　acid　on　NiO(111)　has　been　investigated　but　its　mechanism　is　not　affirmed.　By　using　NIO　＂molecule＂　to　simulate　NIO　crystal,　singlet　potential　energy　curves　for　the　reactions　of　HCOOH--＞CO　+　H2O　and　HCOOH--＞CO2　+　H-2　catalyzed　by　NiO　were　studied　at　B3LYP/6-311G(d)　level　by　the　all-optimum　method　in　this　paper.　The　intrinsic　reaction　coordinate　confirmation　for　two　transitions,　TS2　and　TS3,　was　also　given.　The　results　show　that　formic　acid　adsorbs　dissociatively　on　NiO　and　becomes　formate　HONiOCOH.　The　decomposition　of　formate　is　the　rate-determining　step　for　both　the　reactions,　and　the　activation　energies　of　the　two　reactions　are　almost　the　same.　In　the　dehydration　reaction,　there　is　O-interchange　between　HCOOH　and　NiO,　while　in　the　dehydrogenation,　there　is　no　O-interchange,　which　is　consistent　with　the　experimental　results.　Both　the　decomposition　paths　undergo　a　cyclic　transition　state.　When　HONiOCOH　uses　the　O　atom　of　hydroxyl　(OH)　to　attack　the　H　atom　of　OCOH,　the　dehydration　producing　CO　and　H2O　occurs;　when　HONiOCOH　uses　the　H　atom　of　hydroxyl　(OH)　to　attack　the　H　atom　of　OCOH,　the　dehydrogenation　producing　CO2　and　H-2　occurs.