Integrally skinned asymmetric membranes based on nanocomposite polyethersulfone were prepared by the phase separation process using the supercritical CO2 as a nonsolvent for the polymer solution. All the membranes have been prepared from originally dense nanocomposite films inducing asymmetry by the formation of the porous layer adding N,N-dimethylacetamide as solvent and supercritical CO2 (SC-CO2) to one side of the dense films, and then allowing the supercritical CO2 expansion to occur. The effect of pressure, temperature and nanoparticles on the permeability of CH4 and CO2 and also selectivity performance of membranes has been investigated. The results showed that the membranes formation pressures which varied from 100 to 120 bar have significant effect on the pore sizes and thickness of obtained dense and porous layers. Also, the effect of the temperature which varied from 45 to 55 degrees C has been evaluated and it was concluded that by changing the temperature, it is possible to induce a very-controlled asymmetry in a dense film and pore sizes and thus increasing in CO2 permeability and selectivity performance of membranes. Finally, the permeability of CH4 and CO2 measured at the constant temperatures of 30, 40 and 50 degrees C and at the pressures of 8, 10 and 12 bar and the effects of temperature and nanoparticle on selectivity performance and permeability of gases has been investigated. (C) 2014 Elsevier B.V. All rights reserved.