It was recently discovered that the sputter deposition of metal onto the surface of an ionic liquid generates nanoparticles in the liquid with no additional stabilizing agents. We performed small-angle X-ray scattering (SAXS) experiments to investigate the structure of Au nanoparticles synthesized by this method and to reveal the properties of the ionic liquid that affect the formation process. For a systematic study of these properties, we selected imidazolium-based ionic liquids with different alkyl chain lengths fixing the anion BF 4-. The Au concentration dependence for the formation of the nanoparticles was also studied. The SAXS results revealed that Au nanoparticles with 0.75-3.5 nm diameter were generated under the experimental conditions and that the particle size was relatively uniform for a fixed condition. The results demonstrated that the particle size depends on the type of ionic liquid and on the concentration if the ionic liquid is fixed. Integrating the experimental results, it was concluded that the surface tension and viscosity of the ionic liquid played important roles in conjunction with the Au concentration. It was interpreted that the surface tension influences the initial formation process of nanoparticles on the surface of an ionic liquid, and the viscosity affects the aggregation process during the dispersion of the Au particles from the surface into the liquid.