This paper deals with the combustion behavior and exhaust emissions of a small compression ignition engine modified to operate in diesel/methane dual fuel mode. The engine is a three-cylinder, 1028 cm3 of displacement, equipped with a common rail injection system. The engine is provided with the production diesel oxidation catalyst. Intake manifold was modified in order to set up a gas injector managed by an external control unit.Experiments were carried out at different engine speeds and loads. For each engine operating condition, the majority of the total load was supplied by methane while a small percentage of the load was realized using diesel fuel; the latter was necessary to ignite the premixed charge of gaseous fuel.Thermodynamical analysis of the combustion phase was performed by in-cylinder pressure signal. Gas emissions and particulate matter were measured at the exhaust by commercial instruments. The sizing and the counting of the particles were performed by means of an engine exhaust particle sizer spectrometer. It was found out that dual fuel combustion reduces the carbon dioxide and nitrogen oxides emissions while increases the total hydrocarbons and carbon monoxide. Moreover, it was found out that dual fuel combustion produces lower particulate matter concentration. The effect of methane on particle size and number is influenced by the operating condition. In particular, at high engine speed dual fuel combustion emits larger amount of particles smaller than 20 nm with respect to diesel fuel. The use of a catalytic device allows to reduce considerably the carbon monoxide emissions.