Crystallographic structures of nickel oxide (NiO) ultra-thin films epitaxially grown on ultra-smooth sapphire (0 0 0 1) substrates have been analyzed using synchrotron X-ray diffraction. Growth behaviors of a NiO crystal domain along both an in-plane and an out-of-plane directions were able to be explained at nano-scale resolution. They were drastically changed around 10-15-nm thick film range. Thermodynamic factors on the nucleation and growth was dominant in an ultra-thin film range. On the other hand, the step edges or terrace width of the substrate limited the growth speed in a thicker film range. (C) 2003 Elsevier B.V. All rights reserved.