The aim of this work is to analyze particle number and size distribution from a small displacement Euro 5 common rail automotive diesel engine, equipped with a close coupled aftertreatment system, featuring a DOC and a DPF integrated in a single canning. In particular the effects of different combustion processes on PM characteristics were investigated, by comparing measurements made both under normal operating condition and under DPF regeneration mode.Exhaust gas was sampled at engine outlet, at DOC outlet and at DPF outlet, in order to fully characterize PM emissions through the whole exhaust line. After a two stage dilution system, sampled gas was analyzed by means of a TSI 3080 SMPS, in the range from 6 to 240 nm. Particle number and size distribution were evaluated at part load operating conditions, representative of urban driving.Finally, particle number and size distribution at engine outlet were also evaluated fuelling the engine with neat FAME (Fatty Acid Methyl Ester), in order to evaluate the impact of alternative fuels on PM characteristics during normal operating conditions.Results show that under normal operating conditions the engine and DOC outlet particle number and mass size distributions appeared to be extremely similar, while the DPF exhibited high values of filtration efficiency on a particle number basis also in the nanoparticles range.Regeneration mode caused a particle number increase of an order of magnitude, with a substantial shift of the number distribution peaks towards larger diameters. The particle number across the DOC showed a remarkable reduction for particles larger than 40nm, with one order of magnitude reduction in concentrations.Fuelling the engine with FAME leads to a remarkable reductions both in terms of particle number and mass (up to 80% and 90% respectively) under normal operating conditions.