A small amount of Cu2+ ions was added to the activation bath used for pretreatment of the plating process of an AZ31 Mg alloy. In the activation process, a small amount of Cu was deposited at high density on the substrate surface accompanying Mg dissolution (Cu pretreatment). These Cu deposits acted as nucleation seeds for Zn deposition in the following zincate process and provided a uniform and dense Zn layer almost completely covering the substrate. The Cu layer electroplated on this zincated substrate showed considerable improvement in density and uniformity compared with those of the sample without Cu pretreatment. Cross-sectional SEM observation revealed that a less-defective interface between the Cu layer and substrate was obtained for the Cu-prepared sample. This structure also contributed to the improvement of adhesion strength. The mechanism of this improvement was investigated using electrochemical measurement and scanning electron microscopy observation with energy dispersive X-ray spectroscopy analysis.