Effect of biodiesel blends on the particle size distribution (PSD) of exhaust aerosol and the mechanism of nanoparticle formation were investigated with a modern common rail light-duty diesel engine. The results showed that PSD of diesel included two modes:nucleation mode (NM) and accumulation mode (CM). The criterion diameter of the two modes is 50 nm. Only CM was observed for all fuels under the condition of 50 N.m,2000 r/min. When the engine torque was higher than 150 N.m,log-modal PSD of diesel shifted to bimodal. At higher loads,if the biodiesel blend ratio was below 60%,the PSD of biodiesel blends still included the two modes. However,no NM particles were found for pure biodiesel. At lower loads,only CM was found in PSD of all fuels. Significant reduction of CM particles was found for biodiesel blends compared with diesel. Discussion on the mechanism of nanoparticle formation indicated that for the light-duty diesel engine with oxidation catalysts,fuel consumption and exhaust temperature increased with increasing the engine loads,and SO2 was converted to SO3 by catalyst which,in its hydrated form,could act as the precursor for biodiesel NM formation. Therefore,sulfur level of biodiesel blends dominates the nanoparticle formation in light-duty diesel engine with oxidation catalysts. Effect of biodiesel blends on the particle size distribution (PSD) of exhaust aerosol and the mechanism of nanoparticle formation were investigated with a modern common rail light-duty diesel engine. The results showed that PSD of diesel included two modes: nucleation mode (NM) and accumulation mode (CM). The criterion diameter of the two modes is 50 nm. Only CM was observed for all fuels under the condition of 50 Nm, 2000 r / min. When the engine torque was higher than 150 Nm, log-modal PSD of diesel shifted to bimodal. At higher loads, if the biodiesel blend ratio was below 60%, the PSD of biodiesel blends still included the two modes. However, no NM particles were found for pure biodiesel. At lower loads, only CM was found in PSD of all fuels. Significant reduction of CM particles was found for biodiesel blends compared with diesel. Discussion on the mechanism of nanoparticle formation indicated for for light-duty diesel engine with oxidation catalysts, fuel consumption and exhaust tem SO, the sulfur level of biodiesel blends dominates the nanoparticle formation in light-duty diesel engine with oxidation catalysts.