The purpose of this study is to investigate the mixing stability, fuel properties, and spray-atomization characteristics of diesel-ethanol blended fuels in a common rail diesel injection system. In the diesel blended with ethanol fuel, the most important characteristics are the phase separation and low cetane number of blended fuel. To solve the phase separation, some amount of biodiesel fuel as an additive was added to diesel-ethanol blended fuels. The physical properties of diesel and ethanol blended fuels-such as density, kinematic viscosity, and surface tension-were measured. Based on the fuel properties, the overall spray characteristics were investigated. Spray tip penetration and spray cone angle were obtained from a visualization system. Droplet sizes were measured with a droplet measuring system. The results showed that the addition of biodiesel into the blended fuel prevented the phase separation. With the addition of biodiesel fuel, the diesel-ethanol blended fuels showed no occurrence of phase separation under the closed condition. On the other hand, the increased ethanol blending ratio and the increased fuel temperature resulted in a slight decrease in the spray tip penetration, because of a reduction in spray momentum. However, an increased ethanol blending ratio induced a decrease in the droplet size distribution of diesel-ethanol blended fuels.