The development of defect-free composite membrane (CM) is often challenging due to poor dispersion and distribution of filler particles in the polymer matrix. Despite the attractive physicochemical properties and gas separation performance of carbon nanotube (CNT) based CM, CNT displayed poor dispersion characteristics in most polymer matrix domain. Instead of incorporating CNT, a viable alternative, carbon nanofiber (CNF) which exhibits similar properties as CNT, but improved dispersion quality in the polymer matrix is found. In this work, CNF particles were incorporated in poly(2,6-dimethyl-1,4-phenylene oxide) (PPOdm) polymer continuous phase for CM development. The optimum gas separation performance of the PPOdm-CNF CM (11.25 at 197.02 barrer of CO2 permeability) was obtained at 3 wt of CNF loading. Compared to pristine PPOdm membrane, CO2 permeability and CO2/CH4 selectivity of PPOdm-3 wt CNF CM were enhanced by 180 and 55, respectively. At 3 wt CNF loading, the filler particles were dispersed and distributed more homogenously, in which no obvious CNF agglomeration was observed. In addition, the incorporation of CNF particles also enhanced the mechanical and thermal properties of the resultant CM. © 2018 Elsevier B.V.