We showed a new approach to fabricate separation membranes with enlarged surface areas by nanocasting and investigated the effects of the enlarged surface area on the gas permeability of polydimethylsiloxane (PDMS)-grafted polyimide membranes. A PDMS-grafted polyimide dissolving in solvent was cast onto a patterned nickel (Ni) substrate. The thermal crosslinking reactions occurring among intermolecular PDMS segments allowed the fabrication of insoluble patterned PDMS-grafted polyimide membranes by demolding. The patterned membranes showed higher gas permeability coefficients than the corresponding flat membranes and showed an increasing permeation selectivity for carbon dioxide.