WO3 thin films were fabricated with and without Ar+ plus O-2(+): ion beam assistance on an oxidized Si substrate equipped with interdigitated Pt electrodes and their gas sensing properties were measured. The WO3 film fabricated by ion beam-assisted deposition (IBAD) posessed a finer structure than the film fabricated without the ion beam assistance, and then exhibited high resistance in air and hi,bh response to H-2 due to the grain size effect. The adhesion between the film and the substrate was improved by the ion beam assistance. The thin film sensor fabricated by IBAD kept its extremely high H-2 sensitivity during continuous heating at 300 degreesC for 8 months, whereas the thin film sensor fabricated without the ion beam assistance failed after 2 months. This confirms that the long-term stability of the WO3 thin film sensor can be improved by the simultaneous ion beam assistance.