An experimental investigation was conducted to measure drop sizes in the mixing region of diesel fuel spray. The spray was generated by using an experimental, electronically controlled, common rail type fuel injector. The injector incorporated an intensifier design to generate peak injection pressures as high as 140 MPa. Four different injection nozzles, with orifice diameter ranging from 0.25 mm to 0.40 mm, were used. Background gas density in the spray chamber was varied from about 1.1 kg/m3 to about 42 kg/m3 while the gas was maintained at ambient temperature. Droplets were measured by collection technique as well as by direct photography and were analyzed by computer analyses. Sauter mean diameter was found to decrease with increasing injection velocity for a given nozzle orifice and a given background gas density. The dependence of droplet sizes in the mixing region on orifice diameter as well as on background gas density was seen in the experimental results. Sauter mean diameter was found to correlate well with average injection pressures than with peak injection pressures. The experimental results are used to derive an equation that related non-dimensional Sauter mean diameter to the Reynolds number, gas density and the Weber number.