Abstract: Measurements of particle number emissions and particle size distributions from a heavy-duty truck were performed using a Portable Emission Measurement System (PEMS). The results showed that there were no large changes in particle number concentration over 30 seconds of steady-state operation at different speeds. The results of size distribution of particle number concentration from the truck indicated that the distributions were similar at different vehicle speeds, and showed a unimodal peak distribution. The peak concentration of particles occurred at the diameter of 70 nm except that at idle mode the number was 20 nm. The diameter of particles was greater with the increase of the vehicle speed. There were very few particles with diameter greater than 770 nm, which occupied less than 0.1% of the total.