WO3 nanoparticles were prepared by evaporating tungsten filament under a low pressure of oxygen gas, namely, by a gas evaporation method. The crystal structure, morphology, and NO2 gas sensing properties of WO3 nanoparticles deposited under various oxygen pressures and annealed at different temperatures were investigated. The particles obtained were identified as monoclinic WO3. The particle size increased with increasing oxygen pressure and with increasing annealing temperature. The sensitivity increased with decreasing particle size, irrespective of the oxygen pressure during deposition and annealing temperature. The highest sensitivity of 4700 to NO2 at 1 ppm observed in this study was measured at a relatively low operating temperature of 50 degrees C; this sensitivity was observed for a sensor made of particles as small as 36 nm. (C) 2009 Elsevier B.V. All rights reserved.