Ni thin films were prepared on MgO(100), MgO(110), and MgO(111) single-crystal substrates in the temperature range between 100 and 500°C by UHV-MBE. The growth process, surface morphology, and magnetic properties were investigated. Ni epitaxial thin films were obtained with different film growth orientations depending on the substrate crystal. When Ni films were deposited on MgO(100) substrates, RHEED reflection from hcp-Ni(1120) crystal was observed. However, X-ray diffraction analysis of the samples indicates that the resulting film structure is fcc-Ni(100), suggesting that the hcp-Ni(1120) is metastable and that most of the film has been transformed into a more stable fcc structure. In contrast, Ni epitaxial thin films with (110)fcc and (111)fcc planes parallel to the substrate surface were obtained on MgO(110) and MgO(111) substrates, respectively. The lattice spacings were almost in agreement with the bulk fcc-Ni values for the Ni films prepared on these MgO substrates. The surface morphology influences the magnetic properties of Ni films grown on MgO substrates.