Abstract: By macro examination,chemical composition analysis,metallographic examination,ammonia fume experiment and energy spectrum analysis,the cause of tubercular corrosion on brass tube of condenser was analyzed. The results showed that one of the causes of the tubercular corrosion was the deposit carbon film on inner surface of brass tube,which was created by thermal decomposition of low quality stretching lubricant oil during annealing process of brass tube. Furthermore,neither precoating nor ball cleaning might enhance tubercular corrosion when brass tube came into operation. Doubtless the existence of deposit carbon film was the most important cause of bubercular corrosion. And countermeasures against tubercular corrosion were presented.