The Effect of Space Velocity(SV) on NOx conversion rate was performed to develop NOx reduction after treatment system. Space Velocity is calculated from engine exhaust gas volume and SCR Catalyst volume. Found the Urea injection Duty of maximum efficiency for NOx conversion If increase Space Velocity, NOx Conversion rate is down. Especially, When space velocity is more than 140,000h?¹, NOx conversion rate decrease suddenly. Same case, If Space velocity is lower than 30,000h?¹ NOx conversion rate is down.