3,5-Dimethyl-nitrosopiperazine　(DMNP)　is　a　kind　of　cyclic　nitrosamine　which　was　recognized　as　environment　carcinogens.　The　carcinogenic　potency　of　DMNP　was　observed　to　be　different　when　their　N　atom　was　substituted　with　different　groups.　This　distinction　in　the　metabolic　activation　of　α-　and　γ-position　was　reported　to　contribute　to　their　distinct　carcinogenicity.　In　this　work,　ab　initio　computations　were　carried　out　to　study　the　carcinogenic　mechanism　of　α-　and　γ-positions　of　DMNP　and　its　derivatives.　The　results　showed　that　the　increase　of　the　γ-position　activity　induced　the　decline　of　the　carcinogenic　potency.　However,　the　influence　of　the　α-position　activity　was　ambivalent,　the　carcinogenic　potency　can　be　reduced　either　too　high　or　too　low　activity　on　α-position.　This　study　of　the　metabolic　processes　of　the　α-　and　γ-positions　provided　a　theoretical　evidence　for　the　carcinogenic　mechanism　of　cyclic　nitrosamines.　©　2008　IEEE.