This paper presented results of a study on emission characteristics of diesel engines. A numerical simulation model for a diesel engine was established by GT-POWER. Emission species studied include of NO, CO and HC. The developed model was validated by engine tests under laboratory condition. Based on the model, the simulation changing the variable parameters including injection timing, intake air temperature and EGR (exhaust gas recirculation) ratio were carried out to study their effect on emissions. The simulation results showed that with the decrease of CA BTDC, intake air temperature, compression ratio and EGR ratio respectively, the NO emission decreased. However, the CO and HC emissions increased.