Membrane gas separation is a potential energy-efficient alternative to traditional heat-driven separation processes such as distillation. Mixed-matrix-membranes (MMMs) based on metal-organic-framework fillers in polymeric matrix has many advantages compared to classic polymeric membranes including better performance and tenability. Our group prepared MMMs using three different MOF fillers to boost membrane performance via three different routes. Multi-dimensional MOFs, HKUST-1, has one-dimensional branches at a nanoscale level as well as three-dimensional hierarchical structure at a microscale level. Compared to traditionally spherical MOFs that usually have a single dimensionality, multi-dimensional HKUST-1 form a percolation network in mixed-matrix membranes (MMMs) at a relatively low MOF loading leading to an effective gas permeability higher than theoretical permeability form the Maxwell model. Additionally, the multi-dimensional HKUST-1 MMMs show a significant improvement in CO2 plasticization resistance due to the hierarchical network preventing polymer chain relaxation at high CO2 gas feed pressure. Post-synthetic modification can be easily applied to MOFs such as UiO-66-NH2 due to the presence of the reactive amine functional group. By grafting a short oligomer coating layer on MOF surface, interfacial compatibility between the MOF fillers and the polymer backbone can be enhanced profoundly. As a result, defect-free membrane with high MOF loadings up to 40% can be prepared easily and the resulted MMM has both improved permeability and selectivity. Due to the defect-free nature, Maxwell Model can be applied to estimate transport property sets in pure MOF particles, which remains as a challenge since pure MOF membranes are hard to make. MIL-101, on the other hand, has giant pores and introduces additional free volumes to the membrane. Diffusion of small molecules are favored as compared to larger molecules. Permeation tests show that MMMs involving MIL-101 and 6FDA-Durene polymer have a large increase in permeability without any significant loss in selectivity. Similar to UiO-66-NH2, MIL-101 can also be modified with –NH2 group to tune its affinity towards either the polymer matrix or acidic gases such as CO2.