The　study　describes　decoupling　the　effect　of　surface　oxygen　vacancies　and　Ni　particle　size　in　the　CO2　methanation　over　CeO2　supported　Ni　nanoparticles.　The　availability　of　surface　oxygen　vacancies　with　the　interaction　between　Ni　and　CeO2　served　to　appreciably　enhance　the　catalytic　capacity　for　activation　of　CO2,　generating　higher　conversion　rates,　larger　selectivity　to　CH4　and　lower　activation　energies　relative　to　Ni/SiO2.　A　structure-sensitivity　in　the　reaction　over　Ni/CeO2　was　established　without　the　interference　of　different　concentrations　of　surface　oxygen　vacancies,　where　the　turnover　frequencies　of　CO2　decreased　with　increasing　the　Ni　nano-particle　sizes　(8-21　nm).　The　TPSR　and　Operando　FT-IR　analysis　demonstrated　that　formate　was　the　critical　intermediate　for　CH　4　formation.　The　CO　formation　with　smaller　nickel　size　resulted　from　the　carboxyl　species.　In　addition,　the　long-term　evaluations　and　TGA　measurements　revealed　small　Ni　nanoparticles　suffered　a　temporary　loss　of　activity　due　to　the　carbon　deposition.