Abstract：The selective catalytic reduction is one of the most efficient technologies for NOx reduction. The experimental investigation was conducted on a six-cylinder diesel engine with electronically-controlled cam-actuated unit pumps. The particulate mass of the diesel engine is greatly decreased to meet the China stage IV emission norms by a proper modification of the combustion system. In order to reduce the serious NOx emission, a dosing controlling unit with open loop strategy is developed. The urea solution with concentration of 32.5% is induced into exhaust pipe by using the dosing injection system and the controlling unit. Thus, the NOx reacts with ammonia from pyrogenation of urea and then they are transformed into H2O and N2. Because of high rate of NOx conversion, the open loop strategy for dosing of urea solution is sufficient for meeting the China stage IV emission norms. Firstly, the maps of NOx emission, the exhaust temperature, the exhaust flow and the reduction of NOx per unit urea solution were acquired in the engine calibration tests, and saved into the flash memory of the dosing controlling unit. Then one can calculate the quality of urea solution to be induced into the exhaust pipe by using the maps, the current speed and torque percent of the diesel engine as well as the inlet and outlet of the catalyst. Moreover, the performances of NOx conversion, NH3 slip and urea solution consumption of the selective catalytic reduction post-treatment were investigated in the European Steady Conditions (ESC) and European Transient Conditions (ETC). The results show that, NOx conversion over 65.2% and 65.3% can be achieved in the ESC and ETC tests, respectively, while NH3 slip is below 5ppm. On the basis of SOF analysis, the PM reduction is the result of SOF reduction with SCR catalyst. Also, in the analysis of the urea solution consumption in the ETC test, a heavy duty truck with the diesel engine in the test will consume 2 liter urea solution with concentration of 32.5% per 100 kilometers under the controlling of the dosing controlling unit.