Abstract: The hydrolytic deamination reaction mechanism ofcytosine （ C ） has beentheoreticallyinvestigated at the B3LYP/6-31G level.ItisfoundthatthehydrolyticdeaminationreactionofC may proceedin astepwise way ， in which a tetracoordinatedintermediateisformedfirstly bythe hydrolysisreaction ， followed bythe deaminationreactionwith C — N broken and C O formed. Thestudyonthepotentialenergysurfaceshowedthat NH3 moleculeasashuttle facilitates H transferbetween ketoform and enolform ofhypoxanthine. However ， the deamination reaction of C doesnottake place becauseofa higherbarrierfortheopeningsystem ， whichisagreement withtheexperimentalresult.