We demonstrated that the epitaxial ZnO thin films grown by pulse laser deposition have the highest sensitivity for ethanol gas, depending on their thicknesses, in which there is a close relationship between thickness and sensitivity. The optimum thickness of the ZnO thin film was observed to be 50 nm for the highest sensitivity, in which we also calculated the theoretical factors of sensitivity. For the optimum thickness, we obtained the optimum Al-doping level of about 2%, which showed the highest sensitivity of about 120 for 200 ppm ethanol.