Rutherford backscattering spectrometry (RES) channelling and cross-sectional high-resolution transmission electron microscopy (XHRTEM) have been applied to characterize the structure of Ni films grown epitaxially on MgO(001) by biased d.c. sputter deposition. The RES spectra indicate that the Ni films have a high density of lattice imperfections near to the MgO surface, The XHRTEM investigations revealed a lattice expansion in the [010] direction confirming the existence of the slightly distorted cubic lattice of Ni in the vicinity of the substrate surface which was detected by RES channelling measurements. Regularly distributed edge dislocations due to the mismatch of Ni and MgO lattices have been clearly demonstrated by XHRTEM.