Because of regulation of off-cycle emission and introduction of closed-couple SCR system, exhaust NOx purification performance at high temperature regions becomes more important for commercial vehicle in recent years. In the development of this kind of closed-couple SCR system, it is important to understand the SCR characteristics, and clarifying the SCR characteristics through simulation is extremely beneficial. Based on detailed chemical reaction theory, we have developed SCR model with the gas flow converted to 1 dimension. The developed SCR model was capable of predicting the post-SCR gas composition and NOx purification rate. However, it was discovered that ,for high temperature regions, the purification rate prediction was different with gas bench results and this problem came from considered reaction of NH3 oxidation. It was found that the NH3 oxidation reaction rate was slower than SCR reactions rates and SCR reactions were dominant reactions at high temperature regions of the previous model. In this report, based on analysis of the reactions rate, the model was improved by adding not only the reaction in which NH3 changes to N2, but also the reaction in which NH3 changes to NO in the presence of NOx. Simulated results of improved model were in good agreement with gas bench results. The added reactions competed with SCR reactions in terms of NH3 consumption, therefore NOx conversion was decreasing at high temperature regions. In addition to this, for the accuracy verification, we compared simulated results with the engine bench results. It was found that improved model could calculate outlet gas composition and NOx conversion at high load conditions.