N-nitrosation　mechanism　of　amino　acids　mediated　by　N2O3　was　investigated　at　the　B3LYP/6-311+G(d,p)　level.　Proline　and　tryptophan　were　selected　as　nitrosating　substrates　and　their　reactions　with　three　different　N2O3　isomers,　i.e.,　asym-N2O3,　sym-N2O3,　and　trans-cis　N2O3　were　studied　respectively.　The　obtained　results　demonstrate　that　proline　and　tryptophan　are　able　to　be　effectively　nitrosated　by　all　three　N2O3　isomers,　and　the　nitrosation　of　proline　are　somewhat　easier　than　that　of　tryptophan.　Moreover,　the　N-nitrosation　of　proline　and　tryptophan　is　easier　to　proceed　by　two　relative　unstable　isomers　sym-N2O3　and　trans-cis　N2O3　than　asym-N2O3.　The　results　obtained　in　this　work　will　be　helpful　in　better　understanding　the　mechanism　of　N-nitrosation　reactions　of　amino　acids　and　even　the　modification　of　proteins　through　nitrosation.