Blends of propane-diesel fuel can be used in direct injection diesel engines to improve the air-fuel mixing and the premixed combustion phase, and to reduce pollutant emissions. The potential benefits of usinf propane in diesel engines are both environmental and economic; furthermore, its use does not require changes to the compression ratio of conventional diesel engines. The present paper describes an experimental investigation of the injection process for different liquid preformed blends of propane-diesel fuel in an optically accessible Common Rail diesel engine. Slight modifications of the injection system were required in order to operate with a blend of propane-diesel fuel. Pure diesel fuel and two propane-diesel mixtures at different mass ratios were tested (20% and 40% in mass of propane named P20 and P40). First, injection in air at ambient temperature and atmospheric pressure were performed to verify the functionality of the modified Common Rail injection system. Second, injection process was investigated within the engine. In both configurations, images of the injection process were recorded; in particular, the in-cylinder process was visualized through a sapphire window in the piston bowl. The experimental investigation was performed at four engine operating points of the NEDC (New European Driving Cycle). Results show that the penetration of the blend fuel is as long as the diesel one in the first phase of the injection and is shorter than the diesel fuel at the end of the injection. The cone angle of the blend fuel is always higher than diesel because of the sudden evaporation of propane exiting from the nozzle hole. For these reasons, the use of propane-diesel blends in a diesel engine could allow a better homogenization of the air-fuel mixture.