Membrane-based technology has received much attention in the past decades due to attractive features offered. Through significant breakthroughs of this technology, the mixed matrix membrane (MMM) has demonstrated a promising carbon dioxide (CO2)/nitrogen (N-2) separation performance. Among the polymer materials used, cellulose acetate butyrate (CAB) polymer was selected due to its outstanding characteristics, which subsequently could improve the CO2 sorption ability. In this study, MMM was prepared by incorporating the functionalised multi-walled carbon nanotubes (MWCNTs-F) as inorganic material into the CAB polymer matrix (MMM-4F). The CAB membrane (CAB-M) was also synthesised under similar fabrication parameters via the phase-inversion method to determine the CO2/N-2 separation performance. The gas permeation results showed that the CO2/N-2 separation performance increased dramatically from 6.12 +/- 0.09 to 11.00 +/- 1.92 when 4 wt% of MWCNTs-F was incorporated into the CAB polymer matrix. The increase in CO2 permeance within the MMM was due to the embedded MWCNTs-F within the CAB polymer matrix. In this study, the new synthesised MMM proved to possess excellent CO2/N-2 separation performance. However, it could further be improved by manipulating the amount of MWCNTs-F incorporated into the CAB polymer matrix. This is suitable for the gas separation field. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.