Raw multi wall carbon nanotubes (r-MWCNTs) were embedded as fillers inside the polyimide (PI) matrix and PI/r-MWCNTs mixed matrix membranes were fabricated by the phase inversion method. The TEM images and permeation results using helium as test gas showed that r-MWCNTs were generally closed ended and acted as impermeable nano particles. Gas permeation tests using CO2 and CH4 showed that the addition of r-MWCNTs into the dope solution increased the CO2/CH4 separation factor while decreasing the carbon dioxide and methane permeances. When the r-MWCNTs content was increased from 0% to 6wt.%, permeance of CO2 in the flat sheet mixed matrix membranes decreased from 9.15 GPU to 5.49 GPU and CO2/CH4 separation factor increased from 19.05 to 45.75. Identical to flat sheet mixed matrix membranes, the addition of 2wt.% r-MWCNTs into a spinning dope increased the CO2/CH4 separation factor from 46.61 to 72.20. The glass transition temperature of the mixed matrix flat sheet membranes increased with an increase in the r-MWCNTs content. This implies a good segmental-level attachment between the two phases that forms a rigidified polymer region at the polymer/r-MWCNTs interface. FESEM images showed well dispersed r-MWCNTs in the polymer matrix at a loading of 2wt% r-MWCNTs.