Urea water solution (UWS) droplet evaporation characteristics directly affect the conversion and distribution of NH3 in urea based selective catalytic reduction (SCR) system. The UWS droplet temperature is very difficult to be measured directly. Whereas, this piece of research work involves the measurement of droplet temperature by an Omega-K type thermocouple of 127 mu m diameter. According to the temperature changes of the droplet, the evaporation process can be divided into four steps. Droplets heat and mass transfer processes are derived theoretically at high exhaust temperature. The UWS droplet has been placed in a continuous observation test system to investigate its diameter and temperature variations in the aforementioned four steps. The results shown that, this unique method of four steps analysis has more explicitly and better described the UWS evaporation process, hence establishing the basis for the subsequent detailed simulation and monitoring. (c) 2015 American Institute of Chemical Engineers AIChE J, 62: 880-890, 2016