The Urea-SCR system has been widely used for the after-treatment of NOx to meet tighter regulations for the heavy-duty diesel engine. In addition, the long useful life time of heavy duty engines requires the highly durable components of urea-SCR system such as the catalyst and urea-dosing unit.This paper focused on the deactivation of the SCR catalyst for the EURO4 engine. The NOx conversion of field-aged catalysts was monitored with certain mileage accumulation at the engine bench and laboratory reactors. The postmortem analysis of the catalysts has been carried out to investigate the possible deactivation routes of SCR catalysts in the real driving condition. The analysis showed that the SCR catalyst was durable enough to meet the legislations over the useful lifetime of the engine and found that the non-thermal mechanisms such as poisoning were major routes for the deactivation of catalysts while the thermal sintering was shown to be marginal.Based on these results, the rapid aging modes have been considered as the fast development tools that can replace time-consuming and costly field-aging processes.