Iridium thin films are grown by direct-current plasma magnetron sputtering, on MgO single crystal substrates with various surface orientation, i.e. (100), (111) and (110). The surface morphology, the crystalline properties of the films, and the substrate-thin film interface, are investigated by atomic force microscopy (AFM), X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), and high-resolution transmission electron microscopy (HR-TEM), respectively. The results reveal that hetero-epitaxial thin films with different crystallographic orientation and notable atomic scale smooth surface are obtained. From the XRD analysis the following epitaxial relations are obtained: i) (100) Ir || (100) MgO out-of-plane and [001] Ir || [001] MgO in-plane for Ir grown on MgO(100), ii) (110) Ir || (110) MgO out-of-plane and [1-10] Ir || [1-10] MgO in-plane for Ir grown on MgO(110) and iii) (111) Ir || (111) MgO out-of-plane and two variants for in-plane orientation [1-10] Ir || [1-10] MgO and [1-10] Ir || [10-1] MgO , respectively for Ir grown on MgO(111). Because of the large misfit strain (9.7%), the thin films are found to grow in a strain-relaxed state with the formation of geometrical misfit dislocations with a ~2.8 nm spacing, whereas thermal strain is stored upon cooling down from the growth temperature (600 °C). The best structural characteristics are obtained for the (111) oriented films with a mosaicity of 0.3° and vanishingly small lattice distortions. The (100)-and (110)-oriented films exhibit mosaicities of ̴ 1.2° and lattice distortions of ̴ 1% which can be explained by the larger surface energy of these planes as compared to (111).