Fuel depletion as well as the growing concerns on environmental issues prompt to the use of more eco-friendly fuels. The compressed 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. In this latter case, the CNG was direct injected and the gasoline port fuel injected. The study was carried out on a transparent single cylinder SI engine in order to investigate the in-cylinder process by real time non-intrusive diagnostics. In-cylinder 2D chemiluminescence measurements from UV to visible were carried out. Two filters, at 310 and 431 nm, were used to obtain OH* and CH* spatial distribution as well as to evaluate the local air fuel ratio (AFR) in the cylinder. The OH* and air fuel ratio (AFR) spatial distribution influence the in-cylinder soot formation and oxidation. 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. The in-cylinder optical analysis was correlated to exhaust particle emissions. It was observed that particle emissions, in terms of number and size, are strongly related to the AFR distribution in the combustion chamber.