Ab　initio　calculation　is　carried　out　to　study　the　different　supposed　mechanisms　of　DNA　base　alkylation　by　beta-sulphate-nitrosamines　at　RHF/6-31G(d)　and　MP2/6-31G(d)　levels.　Full　geometric　structure　optimization　is　done　for　all　reactants,　intermediates,　products　and　transition　states.　The　activation　energy　and　IRC　are　obtained.　The　results　show　that　the　anchimeric　assistant　effect　promotes　the　alkylation　of　DNA　base　by　beta-sulphate-nitrosamines.　Solvent　calculation　is　carried　out　with　Onsager　model　of　SCRF　method　at　the　same　level.　The　results　indicate　that　the　activation　energy　is　decreased　obviously　in　water.