To filter the toxic gases from the exhaust of the road vehicles SCR system is widely used in automobile manufactures. The main main of this system to convert the NOx contents of exhaust gas into non toxic gases before releasing into air. The efficiency of the SCR system depend upon the conversion ratio and the back pressure due to the system. The main of this research work to find the optimized solution for the same purpose. In SCR system urea water solution is mixed with exhaust gases in order of convert the NOx content of the exhaust into Co2 , H2O and N2, which are none toxic. I have used Star CCM+ and AVL fire CFD software package to simulated this kind of Multi-phage simulation. The simulations are performed to understand the mixing efficiency and back pressure of the different SCR system. The schematic diagram of typical SCr system is presented in the top left figure whereas the ammonia water injection simulation inside the SCR is presented in the bottom left figure. The video shows the injection of simulation inside tube with mixer.