The effect of annealing on the electrochromic (EC) properties of nickel oxide (NiOx, x > 1) films prepared by reactive rf sputtering is studied with electrochemical measurements, optical transmittance analysis, scanning tunneling microscope (STM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Research results show that EC properties of NiOx films can be changed greatly by annealing. The mechanism for the change of electrochromic properties was discussed in details.