A heat pretreatment was developed to modify the microstructure and properties of the phenolic resin-based carbon supports for the preparation of carbon membranes. The pore size distribution, porosity, surface functional groups, microstructure, and morphology of the supports were characterized by bubble pressure method, boiling method, infrared spectroscopy, X-ray diffraction, and scanning electron microscopy, respectively. Furthermore, the optimum preparation conditions of the as-prepared supports were validated by the fabrication of supported carbon membranes. The results show that the heat pretreatment at 280 degrees C is helpful for the supports to tolerate the high temperature of subsequent pyrolysis and improve the adhesion to surface carbon membrane layers. When supported carbon membranes were prepared by the heat pretreated supports, spin-coating of 4 times, and pyrolysis at 650 degrees C, the ideal selectivities of H-2/N-2 and O-2/N-2 can reach 52.8 and 8.0, respectively.