Abstract: Graphene/polyimide carbon membranes were prepared by an in-situ polymerization method, in which the 9,9'-bis(4-aminophenyl)fluorine (FDA), 9,9'-bis(3-amino-4-hydroxyphenyl)fluorine (BisAHPF) and 4,4'- (hexafluoroisopropylidene)diphthalic anhydride (6FDA) were used as the monomers and graphene oxide (GO) as the pore size regulator. FTIR, XPS, XRD, TGA, N₂ adsorption and gas permeability tests were used to characterize the microstructures and properties of GO and the graphene/polyimide carbon membranes. The effect of the GO content on the microstructure and gas separation performance of the carbon membranes were investigated. Results show that the incorporation of GO into the polyimide significantly increases the total pore volume of the carbon membranes, especially the volume of ultramicropores smaller than 0.6 nm, as well as improving the thermal stability of the membranes. Compared with the polyimide carbon membrane without GO, the graphene/polyimide carbon membranes have an obvious increase in the CO₂ selectivity with a high gas permeability and the selectivity increased with GO loading. The gas permeability of pure CO₂ for the graphene/polyimide carbon membrane with a GO loading of 0.5 wt.% is 8 760 Barrer and its CO₂ selectivities against N₂ (CO₂/N₂) and CH₄(CO₂/CH₄) are 52 and 53, respectively, which are 32%, and 39% higher than that of the pure carbon membrane.