A　theoretical　study　has　been　carried　out　for　internal　rotation　of　nitrosoureas　at　the　B3LYP/6-311G*　level　of　theory.　For　each　nitrosourea　compound,　two　ground　state　structures　have　been　found　and　the　E　isomer　is　predicted　to　be　more　stable　than　the　Z　isomer.　Two　transition　state　conformations　for　the　isomerization　have　also　been　obtained　and　the　calculated　results　show　that　the　isomerization　through　TS1　is　easier　than　that　through　TS2.　The　relationship　between　energy　barrier　and　toxicity　has　also　been　investigated.　It　is　concluded　that　the　carcinogenic　potency　increases　along　with　the　decrease　of　rotational　energy　barrier.