Electrochemical sensors using stabilized zirconia and oxide electrodes were tested for detecting NO or NO 2 potentiometrically and amperometrically at high temperature. For a mixed-potential-type NO x sensor of two-electrode structure, CdCr 2 O 4 was found to give fairly excellent NO x sensing characteristics among the perovskite-type and spinel-type oxides tested. The CdCr 2 O 4 -attached device gave a linear correlation between EMF and the logarithm of NO or NO 2 concentration at 500–600°C. The mixed potential sensing mechanism was also verified. In order to improve the sensitivity and selectivity to NO, the above device was modified to three-electrode structure. With a fixed bias voltage being applied between the sensing (oxide) and counter (Pt) electrodes, the voltage between the sensing and reference (Pt) electrodes was measured as a sensing signal. The responses to NO and NO 2 were found to be enhanced and suppressed with a positive bias, respectively. The tubular device with two-electrode structure was tested for amperometric detection of NO x . A CdCr 2 O 4 -attached tubular device was found to show quick and selective response to NO over NO 2 at 500 and 550°C, if the oxide sensing electrode was polarized at +100 mV versus the reference Pt electrode. The NO x sensing mechanisms of the three-electrode device were proposed and discussed on the basis of the sensing characteristics obtained.