Heavy-duty diesel engines (HDDE), because of their widespread use and reputation of expelling excessive soot, have frequently been held responsible for excessive amounts of overall environmental particulate matter (PM). PM is a considerable contributor to air pollution, and a subject of primary concern to health and regulatory agencies worldwide. The U.S. Environmental Protection Agency (EPA) has provided PM emissions regulations and standards of measurement techniques since the 1980's. PM standards set forth by the EPA for HDDEs are based only on total mass, instead of size and/or concentration. The European Union adopted a particle number emission limit, and it may influence the U.S. EPA to adopt particle number or size limits in the future.The purpose of this research was to study the effects biodiesel blended fuel and cetane improvers have on particle size and number. In addition, a special interest was taken into the variations in diesel particulate matter (DPM) measurements due to the fuel alteration. The Cambustion DMS-500, a fast particle spectrometer, was used to continuously sample diesel aerosol from a HDDE test-cell dilution tunnel with the engine exercised over transient and steady state test cycles. Samples were taken from the primary dilution tunnel and were further diluted in a secondary dilution system. A rebuilt 1992 Detroit Diesel DDC S60 was tested using petroleum-based diesel fuel, biodiesel blended fuel, and a cetane improving additive (2-EHN).The use of cetane improving additives generally resulted in concentration reductions for all of the measured particle sizes that were affected. The blending of biodiesels with petroleum-based fuels has been known to reduce PM mass-based emissions; and in this research it reduced the particle concentrations in the 6 nm to 56 nm and 100 nm to 487 nm size range and did not affect those of the other sizes during transient testing.