The effects of injection rate modulation, i.e., periodical fluctuation of injection rate, on the spatial dispersion of fuel droplets and inner structure of fuel spray were investigated by using an electronically controllable fuel injection system. The major experimental parameters were the modulation frequency and the modulation amplitude. It was confirmed there are two mechanisms of wider spray, i.e., the effect of larger maximum injection rate and the effect of high frequency modulation itself. From the temporal movement of fuel droplets obtained by high-speed photographs, the reason of this phenomenon was discussed.