The emulsion stabilizing properties of soy protein isolate (SPI) were investigated in terms of the protein and oil concentration. Particularly, the dependence of emulsion stability on the oil particle size and viscosity of emulsion was studied in conjunction with the adsorption pattern of protein onto the water/oil interface during emulsification. The data showed that increasing protein concentration decreased the oil particle size and increased the emulsion viscosity, resulting in the enhanced emulsion stability. In contrast, increasing oil concentration increased both the oil praticle size and the emulsion viscosity, and thus emulsion stability varied depending on which factor predominated the overall emulsion system.