Water-in-diesel oil emulsion fuel has been investigated for use in combustion equipment, because of its better fuel economy and pollution reduction effect. In this study, a spray blend mixing system was applied to prepare water-in-diesel oil emulsion fuel, because this system was able to prepare emulsion on-demand. Therefore, the effects of operating conditions such as HLB, gas flow rate, and volume fraction of water phase on the water droplet size were investigated. The water-in-diesel oil emulsion fuel was prepared using the spray blend mixer, and the water droplet size was about several mu m. The water droplet size decreases with increasing gas flow rate or decreasing volume fraction of water phase. In addition, an estimation equation of the average droplet size is proposed.