The sensing performance of a mixed-potential NH3 sensor is dependent on the microstructure of its electrode, which can be optimized to enhance the adsorption and diffusion of target gases. Mesoporous tungsten trioxide was prepared using an effective hard template method and applied as a sensitive electrode in a yttria-stabilized zirconia electrolyte-based mixed-potential NH3 gas sensor. The NH3 detection of the sensor was investigated in a temperature range of 250-650 degrees C. The sensor exhibited a much better NH3 sensing performance than another sensor based on commercial bulk WO3 under the same conditions. Furthermore, the NH3 sensor based on mesoporous WO3 exhibited a logarithmic response to the NH3 concentration (30-250 ppm). To investigate this response, the specialized microstructure of mesoporous WO3 was observed by transmission electron microscopy. Specifically, the high surface area and well-defined mesopores of the material significantly contributed to the excellent sensing performance. This report provides the first demonstration of the excellent response and sensitivity of an NH3 sensor based on mesoporous WO3 at low temperatures (250-400 degrees C).