NiO films were grown epitaxially on MgO (100) substrates by a sol-gel spin-coating technique using a solution of nickel acetate tetrahydrate in 2-methoxyethanol stabilized by monoethanolamine. X-ray diffraction, high-resolution transmission electron microscopy, and selected-area electron diffraction analyses indicated that the NiO films grew epitaxially on the MgO substrates. X-ray reciprocal space maps around the asymmetric 311 diffraction point revealed that the NiO filmswere in a partially strained state, with the lattice parameter contracted along the direction normal to the surface and expanded along the direction parallel to the surface in each film, caused by a lattice mismatch between NiO and MgO. Strain relaxation was gradually enhanced as the film thickness increased. In addition, the Poisson's ratio of NiO was estimated to be about 0.22 from the lattice parameter changes, which is somewhat lower than the value predicted from the elastic constants of NiO. (C) 2015 Elsevier B.V. All rights reserved.