Nano-sized tungsten oxide (WO3) powder shows excellent sensitivity as a NOx gas sensor. The tungsten oxide powders synthesized by polyvinyl alcohol (PVA) polymer solution method showed either plates or spheres of tungsten nanoparticles with agglomeration. The size of the particles was about 30 nm and the surface areas of the powders fell in 10.5~21.5 m2/g. For NO2 gas sensing test, the synthesized WO3 powders have been operated under various temperatures and gas concentrations. The response time, recovery time and stability were strongly depended on the powder size and morphology. The nano-sized WO3 powders showed relatively fast response and high reproducibility.