Sol-gel　derived　tungsten　oxide　(WO3)　films　have　been　deposited　by　dip　coating　route　using　a　small　amount　of　polyethylene　glycol　(PEG)　introduced　into　peroxotungstic　acid　precursor　sol　as　the　precursor　solution,　followed　by　thermal　treatment　in　air.　The　influence　of　PEG　additive　and　annealing　temperature　on　the　structural　and　electrochromic　properties　have　been　characterized.　The　PEG　plays　an　important　role　in　hindering　the　formation　of　large　crystals　and　inter-connected　polycrystalline　structure,　and　also　plays　the　role　of　a　structure-directing　agent,　providing　an　amorphous　WO3　film　when　annealed　at　300　A　degrees　C.　Compared　with　neat　WO3　film,　the　PEG-modified　WO3　film　exhibits　much　higher　optical　contrast　and　higher　coloration　efficiency.　More　importantly,　WO3/PEG　film　shows　excellent　cycling　stability　because　its　mesoporous　structure　acts　as　a　buffer　layer　to　accommodate　the　volume　change　brought　by　ion　insertion/extraction,　and　therefore　the　film　can　expand　and　shrink　much　more　easily　without　causing　degradation　of　WO3　in　long-term　electrochemical　cycling.