Abstract: Gaseous pollutant emission from a high-pressure common rail (HPCR) diesel truck and a diesel truck equipped with an EGR system were tested under real-world driving conditions using SEMTECH-DS, a portable emission measurement system (PEMS). NO_x and THC emission rates within every speed range, integrated emissionfactors under real-world driving conditions, and instantaneous NO_x emission concentrations were compared for the two heavy duty diesel trucks with different technologies. Results showed that average emission rates of NO_x and THC of theHPCR diesel truck were lower than those of the diesel truck equipped with the EGR system in every speed range except for >10-20 km/h. The average NO_x emission concentration of the HPCR truck was lower, but its peak concentration jumped to a higher value. On the contrary, the average NO_x emission concentration of the truck equipped with the EGR system was higher, but its peak concentration was lower.