A sensor of thick film NiO doped with alkali ions was fabricated and coated with Pt as the catalyst on half of its surface. When this sensor was exposed to air mixed with hydrogen gas, the catalytic reaction heated up the Pt-coated surface, and then thermoelectric voltage appeared across the hot and cold region of the oxide film. At 22°C, the sensor showed the built-up temperature difference and voltage signal of 0.12°C and 0.087 mV, respectively, for the 100 ccm flow of 3% hydrogen/air mixed gas, with the full response time, T90, of 55 s.