Mixed matrix membranes (MMMs) with a thermally rearranged (TR) polymer and zeolitic imidazolate framework- 8 (ZIF-8) were prepared in this study. ZIF-8 contributed to not only enhancing gas permeability, but also promoting thermal conversion of TR polymer membranes. As a result, TR ZIF-8 MMMs can be prepared at lower temperatures compared to the original TR polymers. The thermal rearrangement process also increased gas separation properties as it contributes to a tunable cavity size and hour-glass shape cavity distributions. Moreover, interfacial voids between the polymer phase and ZIF crystals which usually occur in most MMMs disappeared during thermal rearrangement of the TR polymer. As a result, the TR ZIF-8 MMMs showed excellent hydrogen separation properties with a H-2 permeability of 1,206 Barrer, H-2/N-2 selectivity of 22.3, and H-2/CH4 selectivity of 25.7.