Silver particles (Ag particles) were synthesized by the photoreduction of silver perchlorate (AgClO(4)) in water-in-ionic liquid (IL) microemulsions consisting of nonionic surfactant Tween 20, water, and ionic liquids, [1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF(4)]) or 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIm][BF(4)])], mixed with a high-pressure (25 MPa) CO(2). The time evolution of the Ag particle formation by photoreduction was investigated by UV-Vis, cryo-TEM, extended X-ray absorption fine structure (EXAFS), and small-angle X-ray scattering (SAXS) measurements. In the particle formation process, aggregation and precipitation of Ag particles were suppressed under high-pressure CO(2). The average diameters of the metallic Ag particles prepared in water-in-[BMIm][BF(4)] and water-in-[OMIm][BF(4)] microemulsions were estimated from TEM to be 3.2 and 3.7 nm, respectively. SAXS analysis shows that the average diameters of the water droplets, which consisted of ionic precursors of AgClO(4) and Ag particles (or Ag aggregates), were estimated to be about 30-40 nm. In the process of Ag particle formation, the water droplet size under high-pressure CO(2) is more effectively regulated than that under ambient air, thereby preventing Ag particles from aggregating and precipitating.