In order to improve the sensing characteristic of mixed-potential-type planar NO sensor with NiO-SE, a series of ZnO (10 wt%, 20 wt%, 50 wt%) were added into NiO. Among the sensor response testing, ZnO was found to give a significant enhancement in NO sensitivity. The electromotive force (EMF) to 700 ppm NO was maximum (22 mV) for the sensor using the 20 wt% ZnO-added NiO-SE at 400 °C under 10 vol % O2 with N2 balance. It was speculated that the formation of new Ni0.9Zn0.1O phase and the modification of morphology of the NiO-SE layer by the addition of ZnO were the main contributing factors for the improved NO sensitivity of the sensor.