Reactive　nitrogen　oxygen　species　(RNOS)　implicate　damage　in　biological　systems,　especially　leading　to　inflammation,　neurodegenerative　and　cardiovascular　diseases,　and　cancer　by　altering　the　functions　of　biomolecules　through　the　N-nitrosation　and　N-nitration　reactions.　The　mechanisms　of　N-nitrosation　and　N-nitration　reactions　of　ammonia　and　dimethylamine　by　RNOS,　i.e.,　N(2)O(3),　N(2)O(4),　N(2)O(5)　and　ONOOH,　were　investigated　at　the　CBS-QB3　level　of　theory.　The　computational　results　indicate　that　the　N-nitrosation　reaction　prefers　a　concerted　mechanism,　in　which　a　H-abstraction　and　ON-addition　occur　simultaneously,　whereas　a　stepwise　mechanism　(also　called　a　free　radical　mechanism)　is　more　favorable　for　most　nitrating　agents　in　the　N-nitration　reaction,　where　NO(2)　first　abstracts　a　hydrogen　atom　from　the　nitrogen　of　amines　and　then　the　induced　intermediate　reacts　with　NO(2)　once　more　to　form　the　nitration　products.　However,　the　concerted　pathway　is　still　a　feasible　process　for　some　nitrating　agents　such　as　N(2)O(5).　In　addition,　the　relationship　between　the　structures　of　different　RNOS　and　their　nitrosating　or　nitrating　abilities　was　also　investigated.