This work investigates the particulates size distribution of reactivity controlled compression ignition combustion, a dual-fuel concept which combines the port fuel injection of low-reactive/gasoline-like fuels with direct injection of highly reactive/diesel-like fuels. The particulates size distributions from 5-250 nm were measured using a scanning mobility particle sizer at six engine speeds, from 950 to 2200 rpm, and 25% engine load. The same procedure was followed for conventional diesel combustion. The study was performed in a single-cylinder engine derived from a stock medium-duty multi-cylinder diesel engine of 15.3:1 compression ratio. The combustion strategy proposed during the tests campaign was limited to accomplish both mechanical and emissions constraints. The results confirms that reactivity controlled compression ignition promotes ultra-low levels of nitrogen oxides and smoke emissions in the points tested. However, in spite of having similar or lower smoke emissions, the number of particles in some conditions is higher for the reactivity controlled compression ignition than for conventional diesel combustion. 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 particle mass for the reactivity controlled compression ignition mode, as typically done for conventional diesel combustion.