A WO3 thin film with a 1.2 μm thickness was deposited onto several substrates, including unpolished alumina, polished alumina, and silicon, using a thermal evaporating method with Pt meander electrodes and a heater. The WO3 thin films were annealed at 600°C in air for 2 h, thereafter, the microstructure and NO2-sensing properties of the thin films grown on the different substrates were investigated. The WO3 thin film on the unpolished alumina exhibited the highest sensitivity of 10–70 ppm NO2 at an operating temperature of 300°C, whereas the film on the silicon substrate showed the lowest sensitivity of 3 ppm. The crystal structures of the thin films grown on the various substrates were nearly the same, however, their surface roughnesses were very different according to the kind of substrate. The sensitivity of WO3 to NO2 is highly dependent on the roughness of the substrate since this is also the main cause of modifications to the surface morphology of the sensing film.