Chloropicrin　(TCNM)　as　one　of　the　most　frequently　detected　nitrogenous　disinfection　byproducts　(N-DBPs)　has　attracted　extensive　attention　due　to　its　high　toxicity.　Although　much　research　work　on　TCNM　has　been　done,　its　formation　mechanism　during　chlorination　has　not　been　known　clearly　yet.　In　this　study,　TCNM　formation　mechanisms　from　methylamine　(MA)　during　chlorination,　including　N-chlorination　of　MA　by　hypochlorous　acid　to　generate　dichloromethylamine　(DCMA)　first　and　then　oxidation　of　DCMA　to　form　nitromethane　(NM)　and　chloronitromethane　(CNM),　and　finally　TCNM　formation　from　C-chlorination　of　NM　and　CNM,　were　investigated　by　using　the　DFT　method.　The　calculated　results　show　that　in　N-chlorination　of　MA,　2-3　water　molecules　involved　in　the　reaction　facilitate　Cl+　and　proton　transfer　with　the　activation　free　energies　(DGs)　for　the　first　and　second　chlorination　in　the　range　of　4-7　and　1417　kcal　mol(-1),　respectively,　which　are　in　good　agreement　with　the　experimental　results.　Formation　of　NM　and　CNM　proceeds　through　a　series　of　elimination,　addition,　and　oxidation　reactions　with　DGs　of　the　rate-limiting　steps　being　around　34-37　kcal　mol(-1),　and　the　subsequent　C-chlorination　of　methyl　in　NM　and　CNM　by　hypochlorous　acid　is　a　rapid　process　with　DGs　below　7　kcal　mol(-1).　This　infers　that　the　TCNM　formation　mechanism　from　DCMA　is　more　likely　to　undergo　first　N-oxidation　and　then　Cchlorination.　These　results　can　explain　the　experimental　findings　that　the　molar　yield　of　TCNM　from　MA　during　chlorination　is　low　(＜　0.1%)　whereas　that　from　NM　is　rather　high　(similar　to　45%).　This　work　will　be　helpful　to　elucidate　formation　mechanisms　of　all　the　halonitromethanes　during　chlorination.