Due to the negative effects that Diesel aerosols have on human health, interest in knowing the size and concentration of particles emitted by Diesel engines is growing. There is no standard procedure to achieve this aim, which makes it difficult to compare most of the results that have been published on this matter. The most widespread equipment for size distributions measurement in Diesel engines is the scanning mobility particle sizer (SPMS), due to its wide resolution and low response time. However, the SMPS is very sensitive to the high concentration of particles and the high exhaust gas temperature. Taking into consideration these limitations, as well as the level of international knowledge reached on these issues, an experimental study has been made, combining the use of a partial dilution minitunnel and a SMPS. The work has been focused on the proposition of a methodological approach that allowed minimization of the main uncertainties present in this type of experimental configuration such as the dilution ratio, the length of the transfer line and the sampling point in the engine exhaust pipe. This knowledge is very useful during the search for cause–effect relationships between the engine parameters or the fuel properties used for the characterization of the emitted particles. The study was carried out on a typical DI Diesel engine which is used nowadays on European roads, using two steady operating modes, located in the most important zone from the point of view of emissions, on its engine map.