An effective method for reducing NOx emissions from automotive engines is the recirculation of the exhaust gas (EGR) to dilute the induced fresh air charge. EGR systems are currently applied mostly with an open-loop control.A new closed-loop control system for EGR rate has been developed. The most important point of this system is use of the feedback signal based on oxygen density at the intake charge for the engine. This system comprises a newly developed oxygen sensor installed in the intake manifold of the engine, an electronically actuated EGR valve and a digital computer unit. The purposes of this system are; a highly reliable EGR control without age deterioration, and an accurate tracking control that traces a scheduled EGR rate.In the first part of this paper, the zirconia oxygen sensor used at the inlet surge tank of the engine is described; both its construction, and some characteristics in several kinds of model gases. Next, the constitution of our trial EGR rate control system for a 2 liter, 4-cylinder engine with a fuel injection system are described, and the experimental results are presented.