Selective catalytic reduction (SCR) is an effective technique to reduce nitrogen oxides (NOx) from diesel engine emissions. The droplet lifetime of the injected urea water is important for the control of DeNOx performance. In this study, the influence of gas temperature and droplet diameter on the evaporation of droplet urea water and the effect of reducing agent were investigated using a simple reaction model for predicting the deactivation of DeNOx performance. This numerical model gives the following predictions: To promote droplet evaporation and to minimize the dependence on temperature, it is necessary to make the liquid drop diameter smaller. Moreover, urea and NH₃ as reducing agents produced similar NO conversion efficiencies and similar deactivation characteristics.