In this study, the fabrication of tungsten oxide (WO3) nanoparticles in effective size using thermal evaporation method under different conditions was reported. The fabricated WO3 were examined and characterized by X-ray diffraction, and field-emission scanning electron microscope. Our examination showed that WO3 nanoparticles with a monoclinic structure were assembled with large quantity. Both of the crystallite and particle size are increase with increasing the evaporation pressure and substrate temperature. In addition, we check the ability of the fabricated WO3 for nitrogen dioxide (NO2) sensing at different operating temperatures. The gas sensing properties showed highest sensitivity of 204 to 1 ppm NO2 at an operating temperature of 140˚C, which was observed for a sensor made of particles as small as 123 nm. The electrical resistance of WO3 showed a unique thermal behavior of the fabricated sensor.