The　initial　reaction　mechanisms　of　N-(2-　chloroethyl)-N-nitrosourea　(CENU)　decomposition　have　been　investigated　at　the　MP2/6-311+G(d,p)　level.　The　mechanistic　processes　considered　were　the　hydrogen　shifting　from　the　nitrogen　to　the　oxygen　of　the　nitroso　group,　the　oxygen　of　the　nitroso　and　the　carbonyl　groups　nucleophilic　displacing　the　chlorine.　The　computational　results　showed　that　the　energy　barrier　of　retro-ene　reaction　was　lower　in　the　gas　phase　than　that　of　substitution　reactions.　In　the　solvent,　however,　the　energy　values　of　each　barrier　in　these　three　processes　approach　each　other.　It　is　concluded　that　the　CENU　decomposition　in　solvent　can　proceed　via　retro-ene　reaction　and　intramolecular　substitution　reactions.