A compact mixed potential type CO sensor was fabricated by using NASICON (sodium super ionic conductor) and spinet type oxide sensing electrodes. NASICON was synthesized by sol-gel process. The spinet type oxides NiFe₂O₄, ZnFe₂O₄and CoFe₂O₄were prepared by coprecipitation method. The X-ray diffraction (XRD) patterns confirmed that these three sensing electrode materials were well crystallized after sintering at 600 degrees C. The surface topography of various spinel type oxide sensing electrodes was examined to have effect on the sensing performances of the sensors by scanning electron microscopy (SEM). The logarithmical correlations between the EMF and the CO concentration were observed for this type of sensors at various operating temperatures. Among the various examined spinel type oxide sensing electrodes, NiFe₂O₄was found to be the most suitable for detecting CO. The response of the sensor attached with NiFe₂O₄to 1000 ppm CO was as high as about 99 mV at 350 ℃.