Dual-fuel technology has the potential to offer significant improvements in the emissions of carbon dioxide from light-duty compression ignition engines. The dual-fuel (diesel/natural gas) concept represents a possible solution to reduce emissions from diesel engines by using natural gas (methane) as an alternative fuel. Methane was injected in the intake manifold while the diesel oil injected directly into the engine. The present work describes the results of a combined numerical and experimental study on combustion process of a common rail diesel engine supplied with natural gas and diesel oil. In particular, the aim is to study the effect of increasing methane concentration at constant injected diesel amount on both pollutant emissions and combustion evolution. The study of dual-fuel engines that is carried out in this paper aims at the evaluation of the CFD potential, by a 3-dimensional code, to predict the main features of this particular technology. In fact, to better understand the phenomena that take place during the dual-fuel operation (flame propagation throughout the premixed methane-air medium activated by the early self-ignition of the diesel fuel), the fluid-dynamic calculations could be extremely useful. Experimentally, the dual fuel operation was investigated in a single cylinder of an optically accessible engine. Experiments were performed at fixed engine speed. Diesel fuel was injected in the combustion chamber by CR injection system at 800 bar and methane in the intake manifold by commercial PFI system at 5 bar. The effect of different premixed ratio of methane injected in intake manifold was evaluated by thermodynamic and spectroscopic data analysis. The pollutant emissions are measured in terms of PM, NOx, CO and THC, which is mainly constituted by methane unburned hydrocarbons. The combustion presents a quite stable operation. Moreover, via both UV-VIS spectroscopy and digital imaging, the spatial distribution of several species involved in the combustion process was analyzed. In particular, OH radical was recognized via chemiluminescence analysis and its integral intensity from UV digital imaging was compared with the ROHR of the several operating conditions investigated. The start of combustion and its evolution was analysed via experimental data and deep information was acquired by numerical activity.