||We have demonstrated the preparation of asymmetric PMMA (polymethylmethacrylate) porous thin film membranes based on phase inversion induced by gelation. Instead of temperature-based physical gelation, a cured slab of elastomeric PDMS (polydimethylsiloxane) has been used as a solvent absorber. Upon solvent absorption of PDMS from the polymer solution in toluene, the system crosses the sol-gel transition curve, resulting in porous membrane structure with thin, dense skin layer at room temperature. Depending on the initial polymer solution concentration and PDMS contact time, various morphologies could have been obtained. Systematic investigation has been performed on the effect of solution concentration and PDMS contact time. The proposed method can be a great help in fabricating asymmetric polymer membranes, because it does not involve any temperature variation or addition of non-solvents.