The preparation of carbon materials with a molecular sieve function is very challenging due to their noncrystalline structure, but they have broad application prospects in petrochemical industries for the separation of gas mixtures. Herein, we report a novel mechanism of controlling ultramicropore size in carbons at sub-angstrom level, and with starch as a carbon source, successful fabrication of novel carbon molecular sieves (SCMSs), with a pore size distribution centered at 4.78 A that is in-between the kinetic diameters of propylene (4.68 A) and propane (