The　mechanisms　of　the　reaction　of　peroxynitrous　acid　and　tyrosine　were　studied　using　the　density　functional　theory　(DFT)　at　B3LYP/6-311G(d,p)　level.　The　geometries　of　all　the　molecules　were　optimized;　the　harmonic　vibration　frequencies　and　the　energies　were　calculated　as　well.　The　calculation　results　showed　that　stepwise　mechanism　rather　than　concerted　mechanism　was　preferred　for　the　reaction　of　peroxynitrous　acid　and　tyrosine.　The　stepwise　pathway　started　with　homolysis　of　the　HO-ONO　bond　to　discrete　center　dot　OH　and　center　dot　NO2　radicals,　which　then　reacted　with　tyrosine　via　two　different　pathways:　(i)　the　H　atom　of　tyrosine　hydroxyl　was　abstracted　by　center　dot　OH　to　produce　IM3,　which　then　combined　with　center　dot　NO2　forming　the　intermediate　(IM1).　Subsequently,　the　IM1　underwent　further　transformation　leading　to　the　product　of　3-nitrotyrosine.　(ii)　the　center　dot　OH　was　added　to　the　phenol　ring　of　tyrosine　to　produce　the　IM2,　which　then　combined　with　center　dot　NO2　forming　the　intermediate　(IM1).　The　IM1　also　underwent　further　transformation　leading　to　the　product　of　3-hydroxytyrosine.　The　activation　energies　of　the　rate-determining　steps　of　these　two　pathways　were　82.86　and　48.05　kJ　.　mol(-1),　respectively.　This　conclusion　was　in　good　agreement　with　the　corresponding　experimental　data.　Additionally,　effects　of　aqueous　solvation　of　water　on　this　reaction　were　also　investigated　and　the　results　indicated　that　the　reaction　preferably　took　place　in　water.