Abstract: A site investigation on corrosion of bottom plate in a 10 000 m³ oil tank is made. This oil tank, with 17. 85 m in height and 28. 5 m in diameter, was put into operation in 1984 and overhauled in 1997. In June of 2000, holiday-type corrosion at bottom plate was found. The corrosion control measure taken in it was a combination method, that is, coating material with sacrificial anode method. A cause and mechanism analysis on the corrosion occurred under this combination way are carried out. In the light of behavior of coating material, painting quality and proportion mismatch of coating materials, the technical point to these problems is given in the paper.