Zirconia coatings were formed on Co-Cr substrates by electrolytic deposition. The microstructure of electrolytic zirconia-coated films on Co-Cr substrates was examined. According to the results of ESCA, the bonding energies of ZrO2 coating surface layer which changed with the annealing temperature from 400 degrees C, 500 degrees C to 600 degrees C are attributed to amorphous (a), tetragonal (t) and monoclinic (m) structure, respectively. The X-ray diffraction (XRD) of the coatings on the Co-Cr substrates annealed at 400, 500 and 600 degrees C revealed the major crystallization from m through t and then to m+t. However, TEM observations clearly showed that the interface layer of the coatings were nanosize crystallites, first the formation of tetragonal and monoclinic ZrO2 structures. These different phase transformations are mainly due to the different surface energy of ZrO2 coating in air, in bulk or on Co-Cr alloy.