According　to　the　results　of　quantitative　pattern　recognition　for　153　N-nitroso　compounds　(NNCs)　based　on　Di-region　theory,　it　is　put　forward　that　the　esters　formed　from　the　metabolism　of　NNCs　on　..-　or　..-position　could　be　alkylating　agents　of　DNA　bases　with　the　anchimeric　assistance　of　the　N-nitroso　group.　This　viewpoint,　combined　with　the　conception　of　..-metabolism　activation,　can　explain　the　structurecarcinogenic　activity　relationship　reasonably.　Quantum　chemistry　ab　initio　calculations　are　carried　out　to　study　the　activity　of　different　metabolites　of　NNCs　in　direct　alkylation　and　the　anchimeric　assistant　processes.　By　ONIOM　method,　a　QM/MM　calculation　is　carried　out　to　study　the　crosslink　of　DNA　base　pair　by　methylalkylnitrosamines.　Based　on　the　computational　results,　the　quantitative　structure　and　carcinogenic　activity　relationship　of　58　N-nitrosoureas　that　have　got　animal　carcinogenicity　tests　reported　are　studied.