A series of aromatic polyimide membranes were synthesized using direct polycondensation of various diamines containing flexible ether groups and bulky substituents with various aromatic dianhydrides. It is expected to promote the gas permeability of the polyimide membrane and maintain similarly gas selectivity by introduction of bulky groups. It was found that the gas permeability of aromatic polyimide membranes increased and the gas selectivity decreased with increasing fractional free volume and d-spacing. In addition, the gas permeability had good correlation with the โฅ-transition temperature and the thermal expansion coefficient. The bulky hexafluoroisopropylidene group introduced into the polyimide backbone could efficiently promote the gas permeability while the gas selectivity decreased slightly. Because the gas diffusivity coefficient and diffusivity selectivity controlled gas permeability and selectivity, respectively, the diffusivity effect dominated the gas separation behavior. The sorption behavior of aromatic polyimide membrane can be well explained using the dual-sorption model. The experimental data indicated that the aromatic polyimide membrane had excellent oxygen/nitrogen separation performance.