Multi-stage injection diesel spray was investigated to understand the internal flow of a diesel spray. This multi-stage spray consisted of two sprays (we called them the first and second sprays) which were formed by a split injection with a short dwell time. In this paper, we discussed the dynamic behavior of a two-stage injection diesel spray. Especially, we focused on the characteristics of internal velocity and the decay of spray density fluctuation. When the injection rate of a conventional spray increased within an injection period, a spray tip of initially injected fuel was caught up and overtaken by the spray of a following injection. Then the internal structure of a conventional spray greatly depended on the internal spray velocity controlled by the injection rate. Since the second spray penetrated into the first spray, the spray tip motion of the second spray could be considered to be similar to the behavior of an internal spray motion in a conventional spray. We could discuss the behavior of conventional diesel spray by using the data of two-stage injection diesel spray.Two different methods were used for these objectives. The first method used a high speed movie to visualize and record the behavior of a two-stage injection spray. The second method was a correlation analysis of the spray density fluctuations. From the motion pictures, it was found that the second spray developed under an influence of the flow induced by the first spray. Therefore, we could obtain some information about the internal flow of the first spray by analyzing the penetration behavior of the second spray. The split injection provided the large momentum and density fluctuations in the spray. By using the light intensity measurement system and FFT analyzer, the decay of density fluctuations or turbulent motions in the two-stage injection spray was investigated. The density fluctuations in the spray were detected as the Mie scattering light from heterogeneous spray cloud. Auto-correlation functions of light intensity signal were derived to estimate the decay of density fluctuations. It was found that the initial fluctuation provided by the injection rate control was rapidly decayed by the turbulent mixing.