It is known that mixed‐matrix membranes (MMMs) suffer from poor compatibility between the polymer and inorganic phases. As a result, suboptimal morphology of a membrane is obtained. This study fabricated MMMs containing ZIF‐L as the inorganic filler and polyethersulfone (PES) via the thermally induced phase separation method. ZIF‐L loading was varied from .25 to 10.0 wt% to study the effect of ZIF‐L loading on the characteristic of the MMMs and gas separation performance (CO2/N2 and CO2/CH4). Unmodified ZIF‐L resulted in significant permeability and selectivity trade‐offs. To improve the compatibility between the components and affinity towards CO2, ionic liquid (IL) was introduced. Two methods of IL modification methods were tested. They are pre‐modification of ZIF‐L and post‐modification of PES/ZIF‐L MMMs. The findings revealed that the pre‐modification method has successfully sealed the polymer/filler interfacial void compared to post‐modification. It was found that the pre‐modification method has enhanced PES/ZIF‐L membrane's performance due to the minimal interfacial defects issue at low loading; 0.75 wt% of pre‐modified ZIF‐L in the PES matrix has enhanced the permeability of CO2 by more than eight times. Furthermore, the ideal CO2/N2 and CO2/CH4 selectivity recorded at this condition is 30.1 and 21.8, respectively. On the contrary, the post‐modification strategy turns out to be counterproductive.