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ZnO nanoparticles were synthesized by flame spray pyrolysis (FSP) using zinc naphthenate as a precursor dissolved in toluene/acetonitrile (80/20 vol%). The particle properties were analyzed by XRD, BET, and HR-TEM. The sensing films were produced by mixing the particles into an organic paste composed of terpineol and ethyl cellulose as the vehicle binder and were fabricated by doctor-blade technique with various thicknesses (5, 10, 15 mum). The morphology of the sensing films was analyzed by SEM analysis. The gas sensing characteristics to ethanol (25-250 ppm) were evaluated as a function of film thickness at 400degC in dry air. The relationship between thickness and ethanol sensing characteristics of ZnO thick film on Al2O3 substrate interdigitated with An electrodes were investigated. The effects of film thickness, as well as the cracking phenomenon, though, many cracks were observed for thicker sensing films. Cracks increased with increasing film thickness because large grains or crack gaps caused the decreasing in the surface area, connectivity of films and deteriorating film properties of the electronic materials. The sensing characteristics with various thicknesses were compared, showing the tendency of the sensitivity to ethanol decreased with increasing a film thickness and a response time. The relationship between gas sensing properties and film thickness was discussed on the basis of diffusively and reactivity of the gases inside the oxide films. The sensing film of 5 mum thick showed the highest sensitivity and the fastest response time (within seconds).