Recently, many kinds of coated steels have been applied for automolile bodies due to their anti-corrosiveness. However, it is very important to control interfaces between phosphate films and electrodeposition films, or sometimes between phosphate films and coated steels, to maximize the properties of coated steels. Cationic electrodeposition has given an unprecedented rise to coating process of automobils bodies. With this method, interfacial control between phosphate films and electrodeposition films becomes a key point because of pH increase in cathodic body area during electrochemical reactions. Galvannealed steels are apt to generate craters in electrodeposition films according to voltage-current program. To suppress the film defect in itself is a kind of interfacial control between phosphate films, electro-deposition films and coated steels. In this paper, studies of interfacial control were carried out.