Tungsten trioxide thin films have been deposited by reactive RF magnetron sputtering on SiO2/Si substrates with different oxygen partial pressure in the Ar–O2 gas mixture. After deposition, the oxide films were annealed in air at 400 °C during 1 h. The morphology and the structure were investigated by Atomic Force Microscopy and Transmission Electron Microscopy. The morphology and the porosity of post-growth annealed films are greatly affected by the oxygen pressure during deposition: the higher the oxygen pressure is, the lower the grain size is. Films with small grains have large roughness and porosity compared to the films with large grains. To investigate the WO3 films sensitivity to ozone, the electrical conductance was measured at various ozone concentration in the range from 0.03 to 0.8 ppm. The sensitivity of WO3 films to ozone depends on grain size and porosity: the sensitivity drastically decreases when the grain size increases.