Reactive magnetron sputtering deposition of nickel oxide (NiO) films were done on p-type silicon substrate and glass substrate under Ar+O2 gas environment at substrate temperature of 26 °C, 100 °C, 200 °C and 300 °C. The phase, crystallinity and grain size were analysed by x-ray diffraction in grazing incidence mode (GIXRD). The average grain size varied in the range of 12 nm to 28 nm for NiO films. The microstructural analysis was carried out under a field emission scanning electron microscopy (FESEM) as well as transmission electron microscopy (TEM). Uniform distribution of grains and lower surface roughness were observed for the NiO films processed at relatively higher temperature. Direct band gap of investigated NiO films were measured with the help of absorption data obtained from UV-visible photospectrometer. The direct band gap was found to vary from ≈2.92 eV to ≈3.96 eV with change in substrate temperature.