Public awareness of the environment and legislation has promoted intensive studies on the reduction of greenhouse gases (GHGs) emission. One of the promising breakthroughs discovered was the polymer blend membrane that demonstrated significant carbon dioxide (CO2)/nitrogen (N2) separation performance. Instead of other polymers, cellulose acetate butyrate (CAB) polymer was chosen due to its high CO2 sorption ability. This membrane was prepared by optimising the ratios of the CAB molecular weights (Mn) of 12000, 70000 and 30000 to enhance the CO2/N2 separation. The single gas permeation test results showed that the CO2/N2 separation performance had increased from 0.7396 ± 0.0396 (M3) to 1.8986 ± 0.0195 (M6) due to the presence of the hydroxyl (OH) group in the membrane. In this study, the newly fabricated polymer blend membrane demonstrated an improvement in the CO2/N2 separation performance. Therefore, this membrane qualifies to be applied in the industry.