The gas permeation behavior of 2, 2'-bis (3, 4'dicarboxyphenyl) hexafluoropropane dianhydride(6FDA)- 2, 4, 6-Trimethyl-1, 3-phenylenediamine (TMPDA)/1,3-phenylenediamine (mPDA) polyimides was investigated by systematically varying the diamine ratios. The physical properties of the copolyimides were characterized by IR, DSC and TGA. All the copolyimides were soluble in most of the common solvents. The gas permeabilities and diffusion coefficients decreased with increasing mPDA content; however, the permselectivity of gas pairs such as H-2/N-2, O-2/N-2, CO2/CH4 was enhanced with the incorporation of mPDA moiety. The permeability coefficients of H-2, O-2, N-2, CO2 and CH4 were found to decrease with the increasing order of kinetic diameters of the penetrant gases. Moreover, all of the copolyimides studied in this work exhibited performance near, lying on or above the existing upper bound trade-off line between permselectivity and permeability.