Ab　initio　calculations　are　carried　out　to　study　the　anchimeric　assistance　of　the　nitroso　group　on　the　β-carbon　electrophilic　center　of　nilramines　and　nitrosoureas.　The　molecular　structures　are　optimized　at　the　B3LYP/6-311+G(d,p)　theoretical　level.　Intrinsic　reaction　coordinate　(IRC)　and　molecular　orbital　analysis　are　calculated　to　investigate　the　reaction　pathways,　and　the　activation　energy　and　the　relative　energy　of　the　intermediates　are　calculated.　Because　the　carbonyl　group　delocalize　the　n　electrons　in　the　molecular　system,　the　negative　charges　on　the　nitroso　oxygen　atom　of　nitrosoureas　are　much　lower　than　that　of　nitrosamines.　The　results　show　that　nitrosamines　are　more　favorable　to　undergo　the　anchimeric　assistant　process　than　nitrosoureas,　which　indicates　that　the　alkylating　activity　of　the　β-electrophilic　center　of　nitrosamines　is　higher　than　that　of　nitrosoureas.　It　is　supposed　that　the　distinct　anchimeric　assistant　activity　of　the　nitroso　group　attribute　to　the　difference　of　the　carcinogenicity　between　nitrosamines　and　nitrosoureas.　©2007　IEEE.