Due to the new challenge of meeting number-based regulations for particulate matter (PM), a numerical and experimental study has been conducted to better understand particulate formation in engines fuelled with compressed natural gas. The study has been conducted on a Heavy-Duty, Euro VI, 4-cylinder, spark ignited engine, with multipoint sequential phased injection and stoichiometric combustion. For the experimental measurements two different instruments were used: a condensation particle counter (CPC) and a fast-response particle size spectrometer (DMS) the latter able also to provide a particle size distribution of the measured particles in the range from 5 to 1000 nm. Experimental measurements in both stationary and transient conditions were carried out. The data using the World Harmonized Transient Cycle (WHTC) were useful to detect which operating conditions lead to high numbers of particles. Then a further transient test was used for a more detailed and deeper analysis. Finally 3-D Computational Fluid Dynamics (CFD) simulations were performed and the numerical results obtained were compared to particle size distributions (PSDs) derived from the experimental measurements carried out in stationary conditions. In this way the influences of engine load and regime on particle size distribution (PSD) were determined. A semi-detailed soot model and a chemical kinetic model, including poly-aromatic hydrocarbon (PAH) formation, were coupled with a spark ignition model and the G equation flame propagation model for the SI engine simulations and for predictions of soot mass and particulate number density. Qualitative agreements of in-cylinder particle distributions were obtained and results are helpful to understand particulate formation processes.