CUI(Corrosion Under Insulation) is worldwide problem. CUI on carbon steel occurs withwater and oxygen and is accelerated by iron rust halogen ion and the conductivity.That mechanism is analyzed more quantitaively to understandthedegree of corrosion influence.By using this mechanism the author applied to the actual plants to find it is correct.But some irregular corrosion occured and is considered why it occurs by means of the investigation of the corrosion samples.