This work investigates the particulates size distribution of reactivity controlled compression ignition combustion, a dual-fuel concept which combines port fuel injection of low reactive/gasoline-like fuels with direct injection of diesel fuel, when implemented in a medium-duty diesel engine. The particulates size distribution measurement was also carried out for conventional diesel combustion at six engine speeds, from 950 to 2200 rpm, and 25% engine load. For this purpose, a scanning mobility particle sizer was used to measure the particles size distribution from 5-250 nm. Both combustion strategies were conducted in a single-cylinder engine derived from a stock medium-duty multi-cylinder production engine with a compression ratio of 15.3. The combustion strategy proposed during the tests campaign was limited to accomplish mechanical as well as emissions constraints. The imposed constraints aim to preserve the hardware of the engine and the feasibility to meet the EURO VI regulation limits in terms of nitrogen oxides. At this point, the pressure rise rate was limited to 15 bar/CAD and maximum in-cylinder pressure to 190 bar. Results confirms that reactivity controlled compression ignition promotes ultra-low levels of nitrogen oxides and smoke, in terms of filter smoke number units, during steady state operating points. However, results suggest that the number of particles measured is higher for the reactivity controlled compression ignition than for conventional diesel combustion, despite of presenting similar or lower values of filter smoke number units. Regarding the particle size distribution measurement, the results show that nucleation mode dominates the particle formation for the reactivity controlled compression ignition mode, while accumulation mode dominates the particle formation for conventional diesel combustion. Thus, it is confirmed that the smoke measurement in filter smoke number units cannot be used to correlate the total amount of particles for the reactivity controlled compression ignition mode, as typically done for conventional diesel combustion.