To clarify the atomization characteristics of transient diesel spray droplet size distribution of spray was measured and Sauter mean diameter (SMD) was also calculated using the optical method based on Fraunhofer diffraction theory at different locations along the spray axis and at different time from the start of injection. To investigate the effects of various operating parameters on drop sizes the injection pressure and the density of gas phase into which spray injected was varied.At each location of spray the timewise measurement of SMD showed its peak value at initial stage and gradually decreased to reach an almost constant value noting that the fully developed spray and droplet - gas equilibrium were being approached. This indicated that the atomization of spray was not a process that ended as the liquid left the nozzle exit but one that continued in time and space. The high SMD value in the initial stage suggested that the leading edge of spray was mainly composed of larger droplets than the trailing spray. This was mainly due to the collision and coalescence between the leading droplets, which undergo large drag by surrounding gas and rapidly slow down, and the succeeding ones.SMD and the probability of large droplets increased as spray proceeded downstream from the nozzle exit because the decrease in droplet velocity and the coalescence between droplets became more dominant than breakup. As expected, SMD decreased drastically as the injection pressure was raised from 10 to 30 MPa since the high injection velocity induced by the high injection pressure promoted the atomization process.The effects of ambient gas pressure on droplet size distribution were also studied. SMD increased as the ambient gas pressure increased.The increase in the ambient gas pressure caused the increase in the gas viscosity resulting check of atomization by air entrainment into spray and consequently increases the fuel ligament diameter. Further, the probability of droplet agglomeration increases due to the reduced penetration and relatively increased number density of droplet.