Fuel depletion as well as the growing concerns on environmental issues prompt to the use of more environmental friendly fuels. The natural gas (CNG) is considered one of the most promising alternative fuel for engine applications because of the lower emissions. Nevertheless, recent studies highlighted the presence of ultrafine particle emissions at the exhaust of CNG engines. The present study aims to investigate the effect of CNG on particle formation and emissions when it was direct injected and when it was dual fueled with gasoline. The study was carried out on a transparent small displacement single cylinder SI engine. The engine was fueled with CNG and gasoline, both simultaneously and not. In particular, CNG and gasoline were direct injected in the combustion chamber. For dual fuel configuration, instead, the CNG was direct injected and the gasoline port fuel injected. In-cylinder 2D images of flame evolution were detected. The flame front propagation was calculated. The in-cylinder soot evolution was evaluated as well. The exhaust emissions were characterized by means of gaseous analyzers and an opacimeter. The particle size distribution function was measured by an Engine Exhaust Particle Sizer (EEPS) in the size range from 5.6 to 560 nm.