The effect of biodiesel blends (derived from waste oil) on soot and particle size distribution (PSD) was investigated in a light-duty common rail direct injection diesel engine under different load conditions. The experiments included four fuels: baseline diesel (D), B40, B60, and B100. (BXX means a blend of XX vol % biodiesel in diesel). Particle size distribution in exhaust was measured by a scanning mobility particle sizer (TSI SMPS3936) through a partial dilution tunnel. The results showed that as the blend ratio increased, the filter smoke number (FSN) of biodiesel blends decreased significantly. With respect to PSD at the downstream of two diesel oxidation catalysts, an increase of engine loads led to nanoparticle formation in the nucleation mode (NM) and log-modal PSD of diesel and biodiesel blends shifts to bimodal. Biodiesel blends will significantly reduce the particle number of accumulation mode (CM) due to soot oxidation from oxygen content. However, the absence of NM was still found for B100. Nanoparticle formation was dominated by the sulfur content of biodiesel blends in this common rail light-duty diesel engine. This was attributed to the process of S-SO2-SO3-sulfate with the existence of a diesel oxidation catalyst. This conclusion suggested that if the sulfur content of biodiesel blends could not reach to a certain level, the blend ratio will have little influence on the nanoparticle number. Total particle number concentration (TPNC) decreased with the increasing of blend concentration. B100 will result in 2 orders of magnitude reduction of TPNC compared with diesel due to the decrease of particle number in CM and absence of NM. The difference between calculated particle mass and FSN soot mass suggested that it is necessary to consider the effect of sulfate on the effective density of the particle when the exhaust aerosol consists of a large amount of sulfate.