Nickel oxide (NiO) films were deposited on glass and silicon substrates using a Ni target by radio-frequency (RF) sputtering at various working pressure ranging from 5 to 20 mTorr. The films were annealed for 2 h at 400 degrees C in the air ambient. The effect of working pressure on structural, optical and electrical properties of NiO films was investigated in detail. The deposition rate of the films gradually increased with decreasing working pressure. The X-ray diffraction (XRD) presented that the crystal quality and the average crystallite size of annealed films increased with decreasing working pressure. The X-ray photoelectron spectroscopy (XPS) confirmed the presence of Ni2+ and Ni3+ ions along with the main peak and satellite peaks in the NiO films. Raman spectroscopy exhibited the one-phonon and two-phonon vibrations modes corresponding to Ni-O bond. UV-Visible analyses showed that annealed NiO thin films have higher transmittance than deposited ones. The band gap of the films increased from 3.33 to 3.52 eV with decreasing working pressure and further increased after the annealing process. The electrical properties were affected by the varying crystallinity and lattice defects depending on the deposition condition. All annealed NiO films exhibited p-type conductivity. The lowest resistivity (4.2 omega.cm) was obtained from the annealed film after deposition at 5 mTorr.