In the current work, thermal treatment, PDMS coating, and combined thermal treatment with PDMS coating methods were applied over NH2-MIL-53(Al)/cellulose acetate (CA) hollow fiber mixed matrix membranes (HFMMM) for CO2/CH4 and CO2/N2 separation. The results showed that upon thermal treatment at 170 °C, CO2/CH4 and CO2/N2 gas pair selectivities of HFMMM were improved from 9.1 to 9.9 and 7.7 to 8.4, respectively. On the other hand, CO2/CH4 and CO2/N2 gas pair selectivities of HFMMMs were improved from 9.1 to 14.1 and 7.7 to 12.0, respectively, after thermal treatment at 220 °C. Substantial increment in gas pair selectivities of the thermally treated fibers was mainly due to polymer densification, which condensed the voids between polymer and particles. Subsequently, PDMS-coated HFMMMs exhibited gas pair selectivities of 17.0 and 16.2, respectively, which were higher compared to the untreated and thermally treated HFMMM. Highest improvement of CO2/CH4 and CO2/N2 gas pair selectivities were found for combined post-treated HFMMM with selectivities values of 26.6 and 24.9, respectively. This result was due to the combined effect of polymer densification and penetration of PDMS solution in the CA matrix which contributed to the suppression of nonselective voids. Hence, this work suggests that combined thermal treatment with PDMS coating method could be feasible and efficient for the improvement of performance of HFMMMs in gases separation. © 2019 American Chemical Society.