Different compositions of carbon molecular sieve (CMS) were incorporated in polyethersulfone (PES) matrix to fabricate mixed matrix membranes (MMMs) by solution casting method. The characterization was carried out using field emission scanning electron microscopy (FESEM) analysis to investigate the morphology of membrane. FESEM images showed acceptable contacts between the filler particles and the polymer chains. The performance of the developed membrane is analyzed by single gas permeation measurement of high purity CO2 and CH4. Both CO2 permeance and CO2/CH4 selectivity increased with CMS loadings as compared to pure PES membrane. Experimental results showed that the highest value of CO2 permeance (66.71 GPU) and CO2/CH4 selectivity (10.94) can be achieved with 30 wt. % loading of CMS particles. This can be credited to size discrimination of CMS pores that falls between CO2 and CH4 kinetic diameters.