Two different carbonaceous nanofillers, carbon nanofiber (CNF) and graphene oxide (GO) were synergistically incorporated into glassy poly (2,6-dimethyl-1,4-phenylene oxide) (PPOdm) polymer to synthesize a novel mixed matrix membrane (MMM) for CO 2 /CH 4 separation. The combined effect of binary filler on the filler dispersion and gas separation performance of resultant membrane were analysed. The binary filler-based MMM rendered homogenous filler dispersion quality, as compared to single filler-based MMM. The CNF particles dispersed in between GO nanosheets, inhibited the restacking of GO whilst the strong steric effect imposed by GO nanosheets improved the dispersion of CNF particles in the polymer matrix. The optimum gas separation performance was recorded at 2 wt CNF/3 wt GO loading with ideal CO 2 /CH 4 selectivity of 59.70 and CO 2 permeability of 62.32 Barrer at 3.5 bar feed pressure. The CNF particles with smooth wall and larger diameter allowed gases to diffuse through faster by creating a gas pathway and eventually improved the gas permeability. Meanwhile, GO nanosheets with surface hydroxyl and carboxyl groups facilitated the transport mechanism of CO 2 molecules, acted as a selective barrier and rendered high selectivity. Therefore, both permeability and selectivity of resultant membrane were enhanced. © Published under licence by IOP Publishing Ltd.