Diesel engine has low carbon emissions and high fuel efficiency. However, diesel engine needs to reduce both Nitrogen Oxide (NOx) and Particulate matters (PM). To meet the demand of strict exhaust gas regulation, after-treatment device is required. Therefore, urea SCR (Selective Catalytic Reduction) system is used to clean NOx in diesel engine exhaust gas. In urea SCR system, it is necessary to inject the urea water solution upstream the SCR catalyst. And, it can reduce NOx applying the generated ammonia (NH3) by urea thermolysis and isocyanic acid (HNCO) hydrolysis. In this study, it focused on urea SCR system. The spray behavior injected in tail-pipe can be divided into the regime of a free spray, an impingement spray, an evaporation of liquid film and a separation droplets, and an urea water solution dispersion. Also, in each region, after evaporation of H2O in urea water solution completely, NH3 is generated by urea thermolysis and HNCO hydrolysis. However, there are cases which biuret (C2H5N3O2) and cyanuric acid (C3H3N3O3) are generated as deposit without generating NH3. The spray behavior injected in the tail-pipe and chemical reaction are very complex, and there are few studies in the conditions assumed for engine. Therefore, the purpose of this study is to reveal the spray behavior and NH3 formation process in the tail-pipe, and to construct the model capable of predicting those accurately. In this report, the urea water spray behavior and NH3 concentration distribution under the high temperature flow field are determined in experimental analysis. Furthermore, the actual phenomenon which greatly influences the improvement of prediction accuracy was identified by comparing with the experimental results using FIRE which is CFD software and the submodel was examined. As a result, NH3 formation process becomes clear, and useful knowledge leading to improvement in prediction accuracy was obtained.