Porous cellulose acetate membranes were prepared by phase separation with supercritical CO2 as a non-solvent. Four kinds of organic solvents were used and the effect of the solvent on the membrane structure was investigated. As the mutual affinity between solvent and supercritical CO2 decreased, the membrane porosity and the average pore size near the center position increased. In all cases, macrovoid was not formed, although the phase separation occurred instantaneously after CO2 was introduced. The membrane performances such as solute rejection coefficient and water permeability A ere measured for the obtained porous membrane.