A planar mixed-potential-type sensor using a yttria-stabilized zirconia and oxide-based sensing electrode (SE) was fabricated and the preliminary results on NH3 sensing characteristics were reported here. It was found that the sensor attached with NiO/Au-SE exhibited good sensitivity and excellent selectivity to NH3 at 800 degrees C under the wet condition (5 vol % H2O). While the cross sensitivities to the other examined gases were about +/- 5 mV or negligible, the sensitivity to 100 ppm NH3 was about -34 mV. The Delta emf of the sensor varied almost linearly with the NH3 concentration in the examined range of 20-100 ppm.