After being injected from liquid rocket injectors or ramjet injectors, liquid propellants are primarily atomized and then accelerated by the combustion gas or the high velocity of coflowing air. During the droplet acceleration process, the motion of the surrounding gas may strongly affect the subsequent process of atomization. Therefore, the characteristics of the atomization may differ from the situation in which injection is made into a still atmosphere. In this study, atomization experiments of impingement-type and coaxial-type injectors were conducted for a flow field with a high-velocity main gas stream. It was found that atomization proceeds while the droplet is being accelerated by the high-velocity gas flow, and that the mean droplet size (SMD) during the atomization process correlates well with the mean acceleration velocity of droplets.