Single-atom　catalysts　(SACs)　have　recently　been　shown　to　have　high　performance　in　catalyzing　the　synthesis　of　NH3　from　N2.　Here,　we　systematically　investigated　a　series　of　single　transition　metal　atoms　anchored　on　stepped　CeO2　(CeO2-S)　to　screen　the　potential　electrocatalysts　for　a　N2　reduction　reaction　(NRR)　via　density　functional　theory　computations.　We　first　demonstrated　that　these　SACs　are　stable　via　large　calculated　binding　energies.　Second,　we　evaluated　the　adsorption　of　*N2　over　CeO2-S-supported　single　atoms.　Here,　those　systems　that　can　activate　N2　molecules　were　selected　as　candidates.　We　then　showed　that　CeO2-S-supported　single　Mo　and　Ru　atoms　have　high　catalytic　activity　for　NRR　via　low　limiting　potentials　of　-0.52　and　-0.35　V,　respectively.　Meanwhile,　the　competitive　hydrogen　evolution　reaction　is　highly　suppressed　over　these　two　SACs　because　the　adsorption　of　*N2　is　prior　to　*H.　Finally,　the　origin　of　the　NRR　activity　over　these　SACs　was　investigated.　This　work　offers　useful　insights　into　designing　high-performance　CeO2-based　electrocatalysts　for　NRR.　Copyright　©　2019　American　Chemical　Society.