In this study, novel kinds of mixed matrix membranes (MMMs) were prepared by incorporating g-C3N4 nanosheets into the matrix of polymers of intrinsic microporosity (PIM-1). The PIM-1/g-C3N4 MMMs were characterized using field emission scanning electron microscope (FESEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction meter (XRD) and an electronic stretching machine. Pure gas permeation tests of the MMMs were conducted for the following gas species: CO2, CH4, N-2 and H-2. Gas permeation properties of the MMMs were rationally tuned with the incorporation of g-C3N4 nanosheets. Firstly, the 2D structural g-C3N4 with high surface area ratio can efficiently affect the packing of PIM-1 polymer chains and create additional transport pathways at the interface between PIM-1 matrix and g-C3N4 filler, which result in an increment of permeability coefficients, especially at low g-C3N4 loadings. Secondly, the periodic ultramicropores of g-C3N4 with size sieving effect can preferentially facilitate the transport of smaller molecules (H-2) and the selectivities for H-2/CH4 and H-2/N-2 were increased without compromise in gas permeability, compared with pure PIM-1 membrane. Meanwhile, the PIM-1/g-C3N4 MMMs also demonstrated better long-term performance with the incorporation of g-C3N4. The ordered 2D-structure, superior interfacial compatibility and easy mass production endow g-C3N4 with promising potential in fabricating gas separation MMMs. (C) 2016 Elsevier B.V. All rights reserved.