Abstract A diesel engine is possible solution for CO2 reduction from automobiles. However, it is necessary NOx-emission-reduction technology for diesel engine. Therefore, this research focused on a Urea-SCR system as a De-NOx system, and proposed the Urea-SCR control based on the the output of an NH3 sensor. The usage of an NH3 sensor is effective not only in De-NOx but also in NH3 slip reduction. Moreover the Urea-SCR control revise Urea dosing amount on the basis of the changes in NH3 slip timing and in concentration in order to compensate for the part-to-part variation in and aging effect of the SCR catalyst. The Urea-SCR system using the NH3 sensor was applied to an actual testing vehicle and achieved higher De-NOx performance and emission robustness than the previous control using a NOx sensor.