Nickel oxide (NiOx) films were deposited onto ITO-coated glass at room temperature by DC magnetron sputtering in Ar/O2 mixing gas. The effect of O2 concentration on structure, morphology, electrochemical and electrochromic properties of NiOx films was systematically investigated. X-ray diffraction results showed NiOx films had the polycrystalline structure. NiOx films deposited at low O2 concentration had the preferred (200) peak. On the other hand, the films exhibited the strong (111) peak at high O2 concentration. Small roughness and grain size of NiOx film deposited at 15% O2 concentration were observed by atomic force microscope and scanning electron microscope results, and small crystallite size was obtained from the XRD data which leads to the good cyclic durability. The large transmittance modulation, high color efficiency and fast coloring/bleaching response time make NiOx films suitable to be applied as an anodic coloring material complemented with WO3 electrochromic window.