The conformation of polystyrene in the anti-solvent process of supercritical fluids (compressed CO2 + polystyrene + toluene) has been studied by small angle x-ray scattering with synchrotron radiation as an x-ray source. Coilto-globule transformation of the polystyrene chain was observed with the increase of the anti-solvent CO2 pressure;i.e. polystyrene coiled at a pressure lower than the cloud point pressure (Pc) and tued into a globule with a uniform density at pressures higher than Pc. Fractal behaviour was also found in the chain contraction, and the mass fractal dimension increased with increasing CO2 pressure.