A practical ZrO2 NOx sensor using dual oxygen pumping cells has been introduced for the control of NOx emitted from a lean-burn gasoline engine and diesel engine.(1),(2). However, the measuring accuracy was not high enough to be useful for controlling or monitoring a low level of NOx concentration such as several tens ppm behind a three way catalyst or lean NOx catalyst which is NOx adsorption or De-NOx catalyst.This paper describes improvement of the interference effect of oxygen in the exhaust gas from the lean-burn gasoline engine and diesel engine. The cause of oxygen dependency is analyzed/revealed and a method of improvement is introduced.The improved NOx sensor has an approximately · · 2% measuring error in the wide range of oxygen concentration on a model gas system, compared to the · ·10% of the previous one. The improved NOx sensor provides a linear signal with a measuring error within · · 10%, in a diesel engine operating under practical conditions in which oxygen concentration, gas temperature, and back pressure of exhaust gas vary from 2 to 14.5 %, 175 to 730 · ·and 2 to 100 mmHg respectively. It is also confirmed that the improved NOx sensor can monitor less than 100 ppm of NOx emitted from a TOYOTA D-4 engine equipped with a NOx adsorption catalyst. The improved NOx sensor would be capable of measuring less than 100 ppm of NOx behind catalyst.Therefore, the improved NOx sensor would be useful for OBD of lean NOx catalyst and for control of NOx adsorption catalyst.