N-nitrosodimethylamine　(NDMA)　as　the　most　frequently　detected　disinfection　by-product　has　aroused　widespread　concern　due　to　its　unusually　high　carcinogenicity,　however,　there　is　still　limited　understanding　of　its　formation　mechanisms.　In　this　study,　the　formation　mechanisms　of　NDMA　from　some　typical　hydrazines　and　hydrazones　with　high　NDMA　conversion　yields　(60%　similar　to　90%)　during　ozonation,　i.e.,　unsymmetrical　dimethylhydrazine　(UDMH),　1-formyl-2,2-dimethylhydrazine　(FDMH),　formaldehyde　dimethylhydrazone　(FDH)　and　acetone　dimethylhydrazone　(ADMH),　were　investigated　by　using　DFT　method　with　the　M05　functional.　A　new　NDMA　formation　mechanism　from　hydrazines　during　ozonation　was　proposed,　in　which　the　initial　step　is　hydrogen　abstraction　rather　than　previously　reported　oxygen　addition.　For　hydrazones,　the　C　atom　of　the-N　=　C　moiety　in　hydrazones　is　preferred　to　be　attacked　by　ozone　to　generate　N,N-dimethylaminonitrene　(DMAN),　which　is　an　important　intermediate　in　NDMA　formation　during　ozonation.　Moreover,　the　reactivity　order　of　the　following　H　atoms　in　hydrogen/hydride　ion　abstraction　(HA)　by　ozone　is　-NH2　＞-N(CH3)(2)　＞-CO-NH　similar　to　=　C(CH3)(2)　＞　=　CH-.　Additionally,　formation　pathways　of　some　experimentally　detected　compounds,　i.e.,　HOOOH,　HOOH　and　HCOH,　in　the　ozonation　of　hydrazine　were　elucidated　in　this　study.　The　results　are　expected　to　expand　our　understanding　of　NDMA　formation　mechanisms　and　ozone　reaction　characteristics.