This study systematically investigates the nanostructural evolution, optical and electrical properties of sputtered nickel oxide films prepared at room temperature. The films with 50 and 100 nm thicknesses were deposited by DC reactive magnetron sputtering. After annealing at 500 degrees C, the films had a polycrystalline (fcc) NiO phase structure. The size of crystallites was from 8 to 28 nm for NiO films with 50 nm thickness and from 7 to 26 nm for those with 100 nm thickness. The grains in both of the annealed NiO thin films exhibited polyhedral morphology with regular and flat edges. It was observed that annealed NiO films were highly transparent, about 70-90% in the spectral range from 500 to 900 nm. A double Schottky barrier was found between two adjacent grains which affects the conductivity of NiO thin films on exposing to different gas atmospheres. NiO thin films with 50 nm thickness exhibit bigger differences of the conductivity to hydrogen in comparison with NiO films with 100 nm thickness in the temperature range from 100 to 250 degrees C. (C) 2016 Elsevier B.V. All rights reserved.