Distinct percentages of carbon molecular sieve (CMS) were added to Polyethersulfone (PES) matrix to generate mixed matrix membranes (MMMs) using solution casting method. The characterization was conducted by thermogravimetric analysis (TGA) to find out the residue solvent in the membranes and field emission scanning electron microscopy (FESEM) analysis to check the morphology of membrane. TGA results demonstrated no remaining solvent and also FESEM images demonstrated acceptable bonds between the filler particles and the polymer chains. The gas permeation results divulged that both CO2 permeance and CO2/CH4 selectivity went up with CMS loadings increment as compared to pure PES membrane. Obtained results revealed that the greatest value of CO2 permeance (68 GPU) and CO2/CH4 selectivity (11.15) at a pressure of 8 bars can be accomplished with 15 wt. loading of CMS particles. This can be related to the kinetic diameter of CMS particles that places between CO2 and CH4 kinetic diameters. © 2016 Published by Elsevier Ltd.