The　conversion　of　peroxynitrite　(ONOO-)　to　nitrate　(NO　3-)　mediated　by　peroxynitrous　acid　(ONOOH)　has　been　investigated　at　the　CCSD/6-311G(d)//B3LYP/6-311+G(d,p)　level.　Two　kinds　of　pathways　for　the　title　reaction　were　found.　The　results　show　that　the　energy　barrier　of　isomerization　through　pathway　1　is　around　25　kcal/mol　in　the　gas　phase.　This　value　is　significantly　lower　than　that　of　isomerization　without　any　catalysts.　Thus,　it　indicates　that　ONOOH　definitely　makes　the　conversion　from　ONOO-　to　NO3-　feasible.　Although　pathway　2　does　not　decrease　the　energy　barrier　of　this　isomerization,　peroxynitric　acid　(O2NOOH)　was　obtained;　moreover,　this　is　a　new　pathway　for　this　formation.　In　view　of　the　results　that　peroxynitrate　anion　can　decompose　into　nitrite　and　dioxygen,　we　conclude　that　our　results　are　consistent　with　the　experimental　observation　that　nitrate,　nitrite,　and　dioxygen　are　the　main　final　products　of　the　decay　of　peroxynitrite　around　pH　7.