The particle number and size distribution are important aspects to qualify diesel engine emissions, considering that new limits, in term of particle number, are expected for Euro 6 regulations. In this scenario it is important to study particulate matter (PM) emissions, not only during engine normal operating. mode, but also during diesel particulate filter (DPF) regeneration processes. The aim of this work is therefore to analyze PM emissions throughout the whole exhaust system of a small displacement Euro 5 common rail automotive diesel engine, during both normal operating conditions and DPF regeneration mode. Because the test engine was equipped with a close-coupled after-treatment system, featuring a Diesel Oxidation Catalyst (DOC) and a DPF integrated in a single canning, the exhaust gas was sampled at the engine outlet, at the DOC outlet, and at the DPF outlet to fully characterize the PM: emissions throughout the exhaust line. After, a two-stage dilution, the sampled gas was analyzed by means of a TSI 3080 SMPS, in the range 6 to 225 nm range. The particle number and size distribution were evaluated at part load, both under normal operating conditions and at DPF regeneration mode, to highlight the impact of the different combustion processes on the PM characteristics. Finally, the particle number and size distribution at the engine outlet were also evaluated while fueling the engine with neat fatty acid methyl ester (FAME) to evaluate the impact of alternative fuels on PM characteristics during normal operating conditions. The results have shown that, under normal operating conditions, the engine and DOC outlet particle number and mass size distributions appear to be very similar, while the DPF exhibits high values of filtration efficiency on a particle number basis, even in the nanoparticle range. Regeneration mode caused a particle number increase of 1 order of magnitude, with a substantial shift of the number distribution peaks toward larger diameters. The particle number across the DOC showed a remarkable reduction for particles larger than 40 nm, with a reduction of I order of magnitude in their concentrations. Finally, fueling the engine with FAME leads to remarkable reductions in terms of particle number and mass (up to 80% and 90%, respectively) under normal operating mode conditions.