Effective and economical carbon dioxide-methane separation (CO 2 /CH 4 ) is highly desirable in several industries such as sweetening natural gases and renewable natural gas (RNG) from biogas and landfills. Among the different separation technologies, membrane separation has been shown to have lower cost of production and lower CH 4 losses. In this study, Zeolitic Imidazole Frameworks (ZIF-8) crystals with sizes varying from 45 nm to 450 nm were synthesized and incorporated in the polymer of intrinsic microporosity, PIM-1, to form mixed matrix membranes (MMMs). The structure, morphology, and physicochemical properties of the MMMs were characterized by 1 H NMR, FTIR, XRD, TGA, and SEM. ZIF-8 crystal size was controlled using the concentration of sodium formate. The influence of the ZIF-8 crystal size on MMMs was studied by sorption, gas permeability, and aging of the membranes. The MMMs with ZIF-8 crystals of 120 nm particle diameter yielded the greatest improvement in gas transport properties; the CO 2 /CH 4 selectivity-CO 2 permeability was 11.4 and 9700 Barrer compared to PIM-1 with 6.4 and 9300 Barrer respectively. The former is near the Robeson 2008 upper bound, while PIM-1 is on the 1991 upper bound. After 40 days of aging, selectivity increased and permeability decreased; the changes were parallel to the Robeson upper bounds indicating increased polymer packing and diffusivity selectivity. • The incorporation of different ZIF-8 crystal sizes in PIM-1/ZIF-8 MMMs showed a significant impact on CO 2 /CH 4 separation performance. • PIM-1/ZIF-8 MMMs with ZIF-8 a crystal diameter of 120 nm improved CO 2 /CH 4 compared to PIM-1, increasing both selectivity and permeability. • The 120 nm diameter ZIF-8 crystal sizes were best dispersed in the MMMs, smaller and larger crystals (i.e., 45, 250, and 450 nm) aggregated and agglomerated respectively, with inferior permeation properties. • PIM-1/ZIF-8B MMMs achieved a good performance, which is close to the Robeson 2008 upper bound. • Physical aging after 40 days decreased the permeability, the selectivity increased as predicted by the Robseon upper bound slope.