Ab　initio　computations　are　carried　out　to　study　the　activity　of　alpha-　and　beta-positions　metabolites　of　nitrosamines.　Full　geometric　structure　optimization　has　been　done　for　all　reactants,　intermediates　and　transition　states.　The　activation　energies,　vibrational　frequencies　and　intrinsic　reaction　coordinate　calculation　are　also　performed.　The　results　show　that　the　activity　of　P-position　is　higher　than　a-position.　For　simulating　the　reaction　in　cellular　environment,　a　series　of　computations　are　performed　in　water　at　the　same　level.　The　results　reveal　that　the　activity　of　P-position　is　enhanced　stronger　than　a-position　by　solvent　effect.　It　can　be　deduced　that　beta-electrophitic　center　may　be　the　first　one　which　alkylates　on　one　of　the　DNA　strands　and　then　alpha-electrophilic　center　may　alkylate.　on　the　other.　These　can　explain　the　alkylation　process　of　nitrosamines　that　would　induce　the　crosslinks　between　the　DNA　complementary　base　pairs　and　initiate　cancer　finally.