Nanosized particles emitted from automotive engines continue to attract concern because of their adverse health effects and their impact on the environment. Automotive engines are a major source of fine and ultarfine particles emitted into the atmosphere. Through stricter emission regulations and the introduction of advanced technologies, the specific particulate mass emissions (in g/km and g/kWh) from internal combustion engines have decreased by about 1 order of magnitude since the 1980s. However, the number concentration of nanoparticles (No./m(3)) emitted from internal combustion engines may continuo to increase considerably and has recently attracted the attention of the Particle Measurement Programme (PMP). This program is intended to evaluate engine nanoparticle measurement systems for use in future emission. regulations. In the study reported in this paper, two latest,generation engines, one spark-ignited and the other compression ignited, were used for a comparison of the particulate emission characteristics, including number density. Both engines were single-cylinder with the same displacement volume of 500 cm(3), and neither included after-treatment traps or catalytic converters. Test fuels used for the study were: gasoline and E85 (mixture, of 85% ethanol and 15% gasoline, sometimes called gasohol) for the spark-ignited engine having a compression ratio of 10; and ultralow sulfur diesel (ULSD) and BD100 100% biodiesel, i.e. soybean methyl ester) for the compression-ignited engine having a compression ratio of 15. A fast-response particle spectrometer (DMS500) with heated sample line was used for continuous measurement of the particle size and number distribution in the size range of 5-1000 nm (aerodynamic diameter). The experimental results showed that particle number peaked within the range of 10-300 nm under till engine operating conditions, regardless of engine combustion type. An observed shift toward large particle size with increasing engine load could be explained by particle coagulation, The effect of the different fuels oil nanoparticle size distributions was dependent on the engine type (spark-ignition or