Non-volatile hydrophilic substances such as ions, proteins, and catalysts have disadvantage that they are insoluble in liquid or supercritical CO₂ because of its very low dielectric constant and polarizability per volume. To overcome this disadvantage, we investigated the formation of the water-in-CO₂ microemulsion that nano size water droplets were dispersed into CO₂. In order to form this microemulsion, the fluorocarbon-hydrocarbon hybrid surfactant, that is, the sodium salt of bis (2,2,3,3,4,4,5,5-octafluoro-1-pentanol) sulfosuccinate (F-AOT) which has CO₂ philic chains and hydrophilic head group simultaneously was synthesized. To investigate the solubility of this surfactant in CO₂, the cloud point was measured with its various concentrations. And "the stability region of microemulsion" was also obtained by measuring the cloud point of water-in-CO₂ microemulsion. It was found that the water-in-CO₂ microemulsion was stably formed in the temperature range from 308.34 to 368.25 K, and in the pressure range from 14.06 to 63.41 MPa.