The emission of greenhouse gases (GHGs), especially carbon dioxide (CO2), has been rising indiscriminately causing serious global warming issues. Therefore, to prevent severe consequences of global warming, stern efforts need to be taken to minimise the global GHG emission. One of the most suitable approaches chosen for removal of CO2 is the gas separation membrane technology. The main objective of this work is to synthesise a membrane with good CO2 permeance and CO2/nitrogen (N2) selectivity that can be applied in the industry. Firstly, the cellulose acetate butyrate (CAB) polymer matrix of molecular weight (Mn) 65000 was used to synthesise the membranes with different solvent evaporation times (4 min, 5 min and 6 min) and different solvent exchange drying times (15 min, 30 min and 60 min). Since the performance of the obtained CAB membrane performance was still relatively low (CO2/N2 selectivity of 0.978 ± 0.011), the mixed matrix membranes (MMMs) were introduced. The MMM was fabricated by using 4 wt% of CAB polymer of molecular weight (Mn) 65000 and 0.1 wt% of functionalised multi-walled carbon nanotubes (MWCNTs-F). In addition, the effect of different MWCNT loadings (0.2 wt% to 1.2 wt%) and acetyl contents (12 wt%–15 wt% and 28 wt%–31 wt%) on CO2/N2 separation were also investigated. The results proved that the MMM prepared from the CAB polymer of Mn 65000 and 0.1 wt% of MWCNTs with higher acetyl content, i.e., from 28 wt% to 31 wt% had better performance in terms of CO2/N2 selectivity (2.887 ± 0.068).