Nitrite　widely　exists　in　the　environment　and　it　was　reported　to　be　a　highly　toxic　to　public　health　when　it　is　in　high　concentration.　Thus,　it　is　of　significance　to　explore　the　ways　to　attenuate　the　nitrite.　In　this　study,　the　mechanisms　of　nitrite　reduction　to　nitric　oxide　mediated　by　iron(II)　porphyrin　were　investigated　by　using　density　functional　theory　(DFT).　Two　pathways　of　the　reduction　of　nitrite　to　nitric　oxide　i.e.,　(a)　direct　conversion　reaction　and　(b)　water-involved　conversion　reaction,　were　proposed.　The　energy　barrier　of　the　direct　reaction　pathway　(a)　was　calculated　to　be　21.0　kcal/mol,　whereas　that　of　water-involved　pathway　(b)　was　13.2　kcal/mol.　It　indicates　that　water-involved　reduction　pathway　is　more　feasible　to　occur.　The　findings　will　be　helpful　to　expand　our　understanding　of　the　mechanism　of　nitrite　reduction　to　nitric　oxide　by　iron(II)　porphyrin.