A modified mixed‐matrix membrane (MMM) was designed in order to improve O2/N2 selective filtration. The fabricated MMM consist of purified and functionalized multi‐walled carbon nanotubes (MWCNT) and polyethersulfone. Characterization of the prepared membranes and the functionalized MWCNT was done by scanning electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis methods. The permeabilities and selectivities of the membranes were improved using modified MWCNT as MMM filler. The effects of the solvent, the carbon nanotube types and dosage, the polymer dosage, the casting method, and the applied pressure in both the membrane fabrication and gas separation processes were investigated.