A Study on the optimal design of the AIG(Ammonia Injection Grid) to improve the performance of DeNOx facilities in the HRSG(Heat Recovery Steam Generator) was performed using the CFD analysis. On the basis of the flow analysis results in the case that the AIG in the HRSG was not installed, the numerical analyses according to the positions of AIG, injection angles of nozzle and the control of ammonia injection quantity were carried out. The standard deviation according to factors was calculated for quantitative comparison. As the results, the AIG in the HRSG should be installed in the position that the uniform flow field shows through the exact flow analysis in the previous of the AIG design and installation. In the case the AIG has already been installed and non uniform flow distribution shows, it is recommended that flow correction device or KoNOx catalyst should be used. Otherwise, the control of ammonia injection angle or the ammonia injection quantity using the velocity profile analysis is demanded to accomplish the optimal performance.