Abstract: This study through the experimental and numerical investigation shows how the intercooler reduces NOx while making the engine performance improved and soot formation decreased. The effects of intercooler on combustion process will be investigated. Also numerical study was performed to investigate the global incylinder characteristics of intercooler system by means of FIRE-CFD code. Results indicated that although the rate of premixed burning increases by using of intercooler due to higher ignition delay period but as a result of lower temperatures in all combustion cycles the NOx emission decreases. Cooling intake air was an effective technique to reduce NOx emission and have shown a reduction of NOx by more than 50% for some operating conditions.