A detailed theoretical study of the structural, magnetic and magneto-optical behaviours of ultrathin films of nickel grown by pseudomorphic epitaxy on semi-infinite Ir(001) is given. The crystalline structure is found to be body centered tetragonal. The total energies are calculated by Spin-Polarized Relativistic Linear Muffin-Tin Orbitals with Atomic Sphere Approximation method. The calculation of the magnetic properties shows a ferromagnetic inter-layer coupling. The polar magneto-optical Kerr effect spectra are calculated over a photon energy range extended to 9 eV. The microscopic origin of the most interesting features is explained by the interband transitions between the localized spin projected states. These transitions are characterized by a spin-flip. (C) 2011 Elsevier B.V. All rights reserved.