Structure and crystal growth of nickel oxide thin films (10-300 nm) prepared by low-temperature sputtering have been investigated by scanning electron microscopy (SEM), X-ray diffraction, and spectroscopic ellipso-metry. Very thin films are compact and homogeneous and are made of almost randomly oriented crystals. A preferential growth direction is then observed following the (111), (220) and (311) planes to the detriment of the (222) and (200) planes, inducing a growth of the materials in columns perpendicularly to the substrate. An optical model able to account for this particular structure has been created from the spectroscopic ellipsometry measurements, and correlates well with the structure observed by SEM. Moreover, it enables an accurate estimation of the thickness without damage to the substrate.