Ultrathin films of YBa2Cu3Oxwith good crystallinity and superconductivity were prepared by a modified off‐axis sputtering. The microstructure, with emphasis on surface morphology and formation of outgrowths, was studied by using atomic force microscopy and electron microscopy. It was found that many outgrowths were formed at the initial growing stage. Therefore it is important to suppress the nucleation of outgrowths at a very early growing step to obtain a smooth film. The nucleation of outgrowths is significantly influenced by surface defects on the substrate. Discussion is also made in correlation with the occurrence of an intermediate layer commonly observed onZrO2. From our observation, YBa2Cu3Oxmight be grown directly on the surface of ZrO2without forming any intermediate layer, if the film thickness is very small. ©1995 American Institute of Physics.