Electrochromic(EC),　nonstoichiometric　NiOx　thin　films　were　made　by　reactive　magnetron　sputtering　at　low　oxygen　flow　ratio(i.e.,　P　=　O-2/Ar　+　O-2　＜　10%).　The　results　of　optical　spectral,　x-ray　diffraction　spectrum,　and　x-ray　photoelectron　spectroscopy　analyses　indicate　that　the　samples　are　oxygen(nickel)-deficient　as　P　＜=　%4(＞=%6),　resulting　in　sub(over)-stoichiometry　films.　Spectroelectrochemical　measurements　show　that　the　EC　effect　of　NiOx　in　nonaqueous　PC　-　LiClO4　electrolyte　is　direct　correlation　with　the　nickel　vacancy　concentration　in　films,　while　that　in　the　aqueous　KOH　solution　is　nearly　uninfluenced　upon　the　change　in　stoichiometry　as　P　＞=　%4.　The　films　deposited　at　P　=　6%　exhibit　higher　coloration　efficiency　of　-　25.3　cm(2)C(-1),　larger　ionic　diffusion　coefficient　of　-　2.84　x　10(-14)　m(2)s(-1),　and　broader　EC　modulation　span　of　24%　in　PC　-　LiClO4　than　the　other　ones.　Based　upon　these　values,　EC　devices　featuring　a　WO3/PMMA　-　PC　-　LiClO4/NiOx　structure　and　excellent　performances　were　fabricated.　We　demonstrated　that　the　nickel　anodization　should　be　responsible　for　the　initial　＂activation＂　phenomena,　which　decreases(increases)　the　number　of　oxygen(nickel)　vacancies.　Moreover,　the　cause　of　degradation　resulting　from　Li+-ion　trapping　in　the　IS　layer　was　also　clarified.　This　work　provides　a　general　framework　for　studying　and　designing　superior　EC　devices,　experimentally　as　well　as　theoretically.