Stable isotopes have been routinely used in chemical sciences, medical treatment and agricultural research. Conventional technologies tonproduce high-purity isotopes entail lengthy separation processes that often suffer from low selectivity and poor energy efficiency. Recent advancesnin nanoporous materials open up new opportunities for more efficient isotope enrichment and separation as the pore size and localnchemical environment of such materials can be engineered with atomic precision. In this work, we demonstrate the unique capability ofnnanoporous membranes for the separation of stable carbon isotopes by computational screening a materials database consisting of 12,478ncomputation-ready, experimental metal-organic frameworks (MOFs). Nanoporous materials with the highest selectivity and membrane performancenscores have been identified for separation of 12CH4/13CH4 at the ambient condition (300 K). Analyzing the structural features andnmetal sites of the promising MOF candidates offers useful insights into membrane design to further improve the performance. An upper limit ofnthe efficiency has been identified for the separation of 12CH4/13CH4 with the existing MOFs and those variations by replacement of the metalnsites.