Mixed potential type NOx sensors were fabricated using YSZ (yittria stabilized zirconia) and oxide sensing materials. Through the simultaneous response to both NO2 and NO gases, sensing behavior of the sensors was studied at a temperature of 700 degrees C in 10% O-2-containing atmosphere. The EMF (electromotive force) was correlated with NO/NO2 concentrations that were measured from the gas analyzer in an effort to elucidate the sensing mechanism of oxide electrode. The possibility of detecting total NOx concentration was proposed.