The main purpose of this research is to investigate high-pressure diesel oil sprays generated by a common-rail system through small nozzle holes. High-speed photography, phase Doppler particle analyzer (PDPA), and a combination of data obtained with high-speed photography and PDPA, based on the light extinction principle within sprays are used to deal with the objective. The sprays are characterised in an environment, which simulates in-cylinder air density of the actual diesel engine when the injection starts. However, it must be pointed out that isothermal condition at room temperature is considered and no-evaporation of drops occurs. A wide parametric study has generated evidence needed to quantify the influence of the common-rail pressure, nozzle hole diameter and environment gas density on the macroscopic evolution of sprays (spray tip penetration, and spray cone angle) as well as microscopic behaviour (spatial and temporal evolution of drop size, drop velocity and drop concentration distributions).