The surface of Zn-Ni alloy electroplated steel sheet is covered by a thin ZnO layer. The ZnO layer has many effects upon degreasing, formation of phosphate film. Furthermore, wet adhesion of a paint film on Zn-Ni alloy electroplated steel sheet is significantly. affected by the ZnO layer and also by the metal ion balance in the phosphate solution. Also, a change in the structure and the phase of phosphate film and, the stress changes in phosphate and paint films occurring during paint film baking and water immersion for wet adhesion testing, were analyzed to study their correlation with wet adhesion. As a result, the mechanism of wet adhesion of a paint film was clarified.