This study focused on the primary breakup of spray at the initial injector opening stage at injection to atmospheric conditions. A real-time mass flow rate instrument was first used to study the injection characteristics at the injector opening stage. A long distance microscope together with an ultrahigh speed CCD camera was then employed to study the primary breakup of diesel spray by photography technique with the help of backlighting. The mechanism of the formation of the mushroom shaped spray head was discussed and the laminar flow regime was expected to be the main reason for the resultant shape. During the initial spray stage, the mushroom spray head penetrated almost linearly with respect to time. The quantification of the fuel mass/spray area ratio suggested that the dispersion of the spray is greatly improved within a very short time, which is assumed to be caused by the further opening of injector. (C) 2015 Elsevier Ltd. All rights reserved.