The influences of different blending percentages of biodiesel on the size distributions of particulate matter (PM) and polycyclic aromatic hydrocarbons (PAHs) are not well known. In this study, commercial pure petroleum-based diesel (D100) and three biodiesel blends of 20% (B20), 60% (B60), and 100% (B100) were tested in an engine operated on a dynamometer following the US transient-cycle test procedure. PM size distributions were measured with micro-orifice uniform deposit impactor (MOUDI) and Nano-MOUDI of 0.01-10 m aerodynamic diameter. The collected samples were extracted then analyzed for PAHs by GC/MS. The results revealed that PM emissions decrease apparently as the blending percentages of biodiesel increase. For D100, B20, B60 and B100, PAH emission factors were 3704, 2720, 1709 and 1514 μg/Hph (horsepower per hour), respectively.Increasing the biodiesel blending percentage reduces the emission of both PAHs and PM. As the blending fractions of biodiesel increased, the PM emissions for the four size ranges decreased. The reductions were significant especially for ultrafine (41.3%) and fine (44.8%) PM. The PAH mass was 32.5%, 32.6%, 34.5%, 30.0% in the ultra-fine size range and 23.8%, 24.3%, 29.2%, 34.5% in the nano size range for D100, B20, B60 and B100, respectively. The addition of biodiesel would cause higher percentages of ultra-fine and nano particulates in exhaust gas. For most biodiesel blending mixtures in the four size ranges, the percentages of PAH emission reduction were higher than those of PM emission. The reduction percentages reached 45.1% and 63.7% for B60, 66.5% and 68.3% for B100, respectively in ultrafine and fine size ranges. The BaPeq emission factors for B100 were 27.2, * Corresponding author. TEL.: +886-423323000 ext. 4451; Fax: +886-423742365E-mail address: hhyang@cyut.edu.tw Chien et al., Aerosol and Air Quality Research, Vol. 9, No. 1, pp. 18-31, 2009 19 49.5, 74.2 and 13.0 g/Hph in nano, ultrafine, fine and coarse size ranges. Biodiesel can reduce both PAH emission factors and the PAH corresponding carcinogenic potency in the full size ranges.