Abstract An experimental campaign was performed to study the behavior of a common-rail Diesel engine in automotive configuration when it is fuelled with blends of Diesel fuel (DF) and waste cooking oil (WCO). In particular the tested fuels are: B20 blend, composed of 20% WCO and 80% DF; B50, composed of 50% WCO and 50% DF; WCO 100% and 100% DF. In order to fuel the engine with fuel having a similar viscosity, this quantity, together with density, has been measured at temperature ranging from rom to about 80C. According to these measurements, before fuelling the engine B20 was heated up to 35C and B50 to 75C. An in-house software was developed to acquire the data elaborated by the electronic control unit. Results show the trend in torque and global efficiency at different gas pedal position (gpp) and different engine speed. The experiments show that larger discrepancies are measured at smaller gpp values, while at larger ones differences become smaller. A similar trend is noticed for engine global efficiency.