In the recent years, growing attention has been focused on internal combustion engines, considered as the main sources of Particulate Matter (PM) in urban air. Small particles are associated to fine dust formation in the atmosphere and to pulmonary diseases. The legislation proposes a stronger restriction in terms of particulate mass concentrations for both Diesel and gasoline engines and a limitation on number concentration. Unfortunately, the experimental evaluation of particles number and size is a hard task as they are strongly affected by the dilution conditions, due to condensation and nucleation phenomena, which may occur during the sampling. Even if a considerable amount of basic research on particulate matter emitted by engines has been carried out, the mechanisms governing particle formation are still not fully understood, neither for Diesel nor for gasoline engines. Furthermore, poor information is available on the effect of engine control parameters and of the emissions control technologies on particles size and number.The aim of the paper is the experimental characterization in terms of number and size of particles emitted from a gasoline engine in steady state operating conditions. The gasoline particles investigation was carried out at the exhaust of a 1.2 liters port fuel injection spark ignition engine. The engine test bench was equipped with a full-pass engine control system dSPACE MicroAutoBox and a Scanning Mobility Particle Sizer (SMPS) for particles counting and sizing, thus allowing accomplishing a detailed analysis on the effects of the engine control variables on particles emission.