By using Eu2CuO4 (ECO) as a buffer layer, extremely smooth surface YBa2Cu3O7-x (YBCO) thin film on yttrium stabilized ZrO2 substrates was obtained. The microstructure of YBCO thin films with or without ECO buffer layer were studied by atomic force microscopy, scanning electron microscopy and X-ray techniques. It was found that the epitaxy and crystallinity and surface of the YBCO thin films have been significantly enhanced with ECO buffer layer. (C) 2003 Elsevier Science B.V All rights reserved.