In this paper a turbocharged aftercooled NG SI engine was experimentally investigated at Sharif University turbocharging lab at WOT. The engine torque, boost pressure, air consumption and volumetric efficiency were assessed experimentally at WOT in view point of waste-gate. Waste-gate opening effects on turbine and compressor characteristics were also evaluated experimentally at Sharif University turbocharger lab. The engine was also experimentally investigated at four different waste-gate opening pressure thresholds under ECE-R49 test cycle. It is also experimentally found that changing waste-gate opening pressure threshold from 165 mmHg to 200 and 265 mmHg decreases total bsNOx at a rate of 6% and 12% respectively. Increase of the pressure threshold to 323 mmHg augmented total bsNOx. Therefore, to minimize the bsNOx, the threshold of 265 mmHg was the optimum pressure threshold. Waste-gate not only had substantial effects on turbocharged CNG SI engine, but also significantly affected on NOx emissions.