The performance of commercial scale selective catalytic reduction (SCR) system is strongly dependant upon the degree of mixing between NH3 and NOx or NH3 concentration distribution at the catalyst layer according to the reaction kinetics of SCR catalysts. Insufficient mixing of the reduction agent and NOx mass flow necessitates an uneconomically large catalyst volume and high NH3 slip to meet the required NOx emission values. The effective methodology which can increase the performance of commercial scale SCR through improving NH3 concentration distribution at the catalyst layer using computational fluid dynamics (CFD) analysis was suggested and applied to the real operations. The operation results have shown that the performance of commercial SCR was improved from 54.4% to 74.8% as NH3 concentration deviation at the catalyst layer was reduced from 23.6% to 8.6%. It is established that the increase of NH3 concentration uniformity at the catalyst layer contributes to improvement of performance of commercial scale SCR.