In a diesel engine, the mixing of the fuel spray and in-cylinder air controls rate of beat release during combustion, namely it will determine the thermal efficiency, maximum output and gas or noise emission, etc.Therefore, it is important to measure the droplet size and its volume density distribution in diesel fuel sprays. The optical measuring method, which includes a light scattering and holographic technique, seems the only feasible method for analysing these unsteady characteristics of fuel sprays.The light scattering technique described herein was based upon Mie scattering theory, and the droplet size and volume density distribution of fuel sprays were calculated from the combination of the light extinction and the forward-to-backscattering ratio of Mie scattering intensity. The volume density and droplet size distribution of fuel sprays were obtained from the light intensity distribution on a photograph of fuel sprays. On the other hand, the in-line pulsed laser holography was applied for observation of droplet size of the fuel sprays, and was compared with the light scattering method. While the light scattering technique is useful for the macroscopic measurement of volume density and droplet size distribution of fuel sprays, the in-line pulsed laser holography enables observing them microscopically. The diesel fuel sprays from several conditions of injection system were analysed by the above-mentioned techniques, and the effects of different specifications on volume density and droplet size distribution of fuel sprays could be clarified.