Particulate matter produced by diesel engines is of concern to cngine manufactures because of its environmental impact. The majority of diesel particles are in the range of smaller than 1 ${mu}{ extrm}{m}$. Because of their tiny volume, ultrafine diesel particles contribute very little to the total mass concentration which is currently regulated for automobile emissions. Ultrafinc particles are known to have deleterious effects upon human health cspecially because they penetrate deeply human respiratory tract and have negative effects on the health. In this study, the engine exhaust gas was diluted in a dilution tunnel and the particle size distribution was measured using the scanning mobility particel sizer system. Measurements of the number and the mass concentrations of the diesel exhaust were made under different engine ooperating conditions. The dilution sampling system provided a common basis for collection of the exhaust by cooling and diluting the source emission prior to the measurement. The measurement results showed that the particle size distributions of the exhaust from the diesel vehicles equipment with either heavy-duty or lignt-duty diesel engines, were similar in the particle size range of 0.08~0.2${mu}{ extrm}{m}$.