Highly selective carbon molecular sieves (CMS) were dispersed in polysulfone (PSU) matrix with various loading concentration (10, 20, and 30 wt%). A thorough investigation of membrane morphology, mechanical, and thermal properties are discussed. Separation performance of the membranes was tested by using CO2 and CH4 gas, respectively, in five different feed pressures within the range of 2-10 bar. The presence of CMS increases the permeability of CO2 and decreases the permeability of CH4. In correspondence, the ideal selectivity of CO2/CH4 is enhanced.