The electrical and optical properties as well as the electronic structure of sodium-doped (Na-doped) nickel oxide (NiO) thin films were investigated using X-ray photoelectron spectroscopy (XPS), reflection electron energy loss spectroscopy (REELS), X-ray absorption near edge structure, and extended X-ray absorption fine structure. The XPS results confirmed the presence of Ni-O bonds of the NiO phase for all films via the Ni 2p spectra. The NiO thin films annealed above 200 degrees C have both nickel oxide and nickel metal phase. The REELS spectra showed that the band gaps of NiO thin films after Na doping were decreased. Na-doped NiO thin films exhibited relatively low resistivity compared to undoped NiO thin films. In addition, the Na-doped NiO thin films deposited at room temperature have p-type conductivity with a low electrical resistivity of 11.57 Omega cm and high optical transmittance of 80% in the visible light region. Our results demonstrate that Na doping plays a crucial role in enhancing the electrical and optical properties of NiO thin films. (C) 2015 Elsevier B.V. All rights reserved.