In order to evaluate corrosive sulfur in insulating oil quantitatively, wave length dispersive X-ray (WDX) technique has been reported 1. Using this technique, antioxidant 2,6-di-tert-butyl-p-cresol (DBPC) was found to be effective to suppress corrosion caused by dibenzyl disulfide (DBDS). The effect of corrosion prevention continues for a long term when a certain amount of DBPC is added to oil. The durability of DBPC was also evaluated by HPLC. It is possible to add relatively a good deal of DBPC to oil because DBPC has higher solubility than passivator and little effect on dielectric strength. Furthermore, DBPC was used with passivator simultaneously, and corrosion control time was far extended by their synergy effect. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.