In this article,the electrochromic (EC) properties of R.F.deposited NiOx thin films are investigated.It was found that non-stoichiometric NiOx(x>1) thin films which are rich in O element have EC activation.In NiOx thin films exist both Ni3+ and Ni2+ ions.The insert ion of H+ ions and their sequential occupation of Ni vacation render t2g energy levels of Ni3＋ ions completely filled.Thus,Ni3+ is reduce to Ni2+ ions and the NiOx films get light transparent.On the contrary,the extraction of H+ ions from NiO x films will produce vacancies in the t２g energy levels o f Ni２＋.Ni2+ ions are oxidized to Ni3+ ions,wh ich makes NiOx films absorb light.The film is then colored.