The aim of this study is to investigate the effect of operating conditions including water content ratio on the particle size of the water phase dispersing in emulsified fuel droplets sprayed from rapid internal mixing injector (RIMI). An experimental apparatus based on immersion method was constructed to collect the emulsified fuel droplets, and capture photographs of water particles. The results showed that the particle size of the dispersed phase decreased rapidly with the increasing velocity of air atomizing emulsified fuel, and then approached a constant value.