In this study, four soluble homo- and co-polyimides using 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) and 4,4'-diaminodiphenyl ether (ODA) monomers were synthesized to developthe gas separation membrane with good CO₂/CH₄separation properties. To prepare the copolyimides, 20 mol% of three dianhydrides- (4,4'-(hexafluoroisoproplidene)diphthalic anhydride (6FDA), 4,4'-biphthalic anhydride (BPDA), 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA) - were added in DOCDA-ODA monomer mixture, respectively. All the synthesizedhomo- and co-polyimides were characterized by FT-IR. Their thermal properties were analyzed with differential scanningcalorimeter (DSC). Dense membranes were prepared from these copolyimides to check their gas permeation propertiesfor CO2 and CH4 gases using a time-lag method. The permeation testing results are as follows; DOCDA/ODA homopolymershowed 1.71 barrer of CO₂ permeability and 74.35 of CO₂/CH₄ selectivity. The three polyimide copolymers (DOCDA/6FDA-ODA, DOCDA/BPDA-ODA, DOCDA/BTDA-ODA) showed lower CO₂/CH₄ selectivities and higher CO₂ permeabilitiesthan the homopolymer (DOCDA-ODA). DOCDA/6FDA-ODA showed twice times higher CO₂ permeabilitieswithout severe CO₂/CH₄ selectivity loss than the DOCDA-ODA.