Effects of sugars on the formation of nanometer-sized oil droplets induced by the addition of vegetable oil to aqueous dispersions of polyoxyethylene sorbitan monooleate (MOPS, Tween 80) at 25 °C without the application of intensive mechanical energy were investigated. Phase diagrams were constructed using polarized light microscopy and small angle X-ray scattering (SAXS) to elucidate the relationship between the type of phases involved in the process of emulsification and the droplet size in the resulting emulsions. Nanometer-sized oil droplets as small as 220 nm in diameter were obtained when the sponge phase (L3) was formed at first, followed by the phase transition to coexisting multiple phases including the micellar cubic phase (I1) with increasing vegetable oil content. Sugars expanded the area of the sponge phase toward lower MOPS contents, enabling the formation of nano-emulsions from a wider range of the initial composition. The area of the sponge phase increased in the order of d-fructose ≈ d-glucose < sucrose < d-maltose, consistent with the order of the literature value of the mean number of equatorial hydroxyl groups per sugar molecule and that of the hydration number of sugar that represents the average number of water molecules forming a complex with a single molecule of sugar in aqueous solution. The present results confirm that sugars facilitate the formation of nano-emulsions using the isothermal low-energy emulsification method, presumably due to their abilities to shift the effective hydrophile–lipophile balance (HLB) of the surfactant toward the hydrophobic side.