The　preparation　of　material　is　critical　to　fabrication　of　gas　sensor　made　from　semiconductor.　Nanometer　WO3　was　synthesized　by　gas-state　reaction　with　tungsten　filament　and　made　into　gas-sensor　to　NO2.　The　gas-sensing　measurements　results　revealed　that　these　sensors　show　a　sensitivity　about　10　when　put　into　NO2　gas　of　very　low　density　(about　10　-10)　and　respond　quickly(about　1.　5　seconds).　By　analyzing　micro-structure　of　the　material,　the　mechanism　of　these　sensors　was　uncovered.