NiO layers of different thickness have been epitaxially grown on a MgO(1 1 0) substrate at various temperatures in the range 700-900 degreesC. Investigations by reflection high energy electron diffraction, atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) give evidence for a roof-like morphology of the NiO(l 1 0) surface consisting in (1 0 0) and (0 1 0) facets elongated along the [0 0 1] direction. The period and height of these facets depend on the layer thickness and the deposition temperature. This particular surface configuration is due to a minimization of the surface energy during the growth process.