The use of diesel engines day by day increased because of their low fuel consumption and high performances. Common rail diesel engines with electronic fuel injection can accurately meter the fuel injection quantity and fuel injection pressure. Injection spray pressures in diesel engines play an important role for engine performance and emissions obtaining treatment of combustion. In this paper, effects of fuel injection pressure and spray cone angle on the exhaust emissions of a national diesel engine have been numerically investigated using a model verified by experimental data. In this study, four fuel injection pressures of ????, ????, ???? and ???? bar and three spray cone angle of ??, ?? and ?? degrees at ???? bar injection pressure were investigated. The simulation results showed that by increasing the fuel injection pressure, the maximum pressure inside the combustion chamber increased from ??? to ??? bar and the temperature inside the combustion chamber increased by ?.?% and changed from ???? to ???? K. Subsequently, the amount of NOx produced also increased. But the amount of soot decreases with increasing fuel injection pressure due to better mixing of fuel and air. Changing the spray cone angle from ?? ° to ?? ° decreased the pressure inside the combustion chamber from ??? to ??? bar. One of the reasons for this decrease was the impact of fuel droplets against the walls of the combustion chamber at higher spray cone angles, this causes the fuel to not completely vaporize. An increase in the temperature of the combustion chamber was observed at a cone angle of ?? ° C relative to ?? ° C from ???? to ???? ° C. An increase in the temperature of the combustion chamber was observed at spray cone angle of ?? ° relative to ?? ° from ???? to ???? K.