During　the　metabolic　process　of　Methylated　Polycyclic　Aromatic　Hydrocarbons　(PAHs),　there　is　a　debate　on　the　theory　that　which　one　is　the　＂ultimate　carcinogen＂　between　the　positive　ion　of　angular　ring　and　methylated　derivatives.　In　this　paper　a　systematic　calculation　(ab　initio　and　Density　functionary　theory)　on　methylated　B[a]A　has　been　carried　out　to　acquire　the　good　agreement　with　the　experiment　according　to　the　model　of　metabolism.　The　computational　result　shows　that　the　easiness　of　protonated　trend　on　the　two　eletrophilic　center,　together　with　its　stability,　is　essential　to　the　mutagenicities　and　carcinogenicities.