The direct synthesis of oriented, defect-free nanocrystal metal–organic framework (MOF) films is a challenging step toward their applications in advanced technologies, such as optics, sensing, and membrane-based separations. Here, we propose a one-step, in situ growth approach to synthesize oriented zeolitic imidazolate framework-L (ZIF-L) membranes by using an isoporous film as the support. The high metal binding efficiency, as well as the ordered pore structure, given by the polymeric isoporous support, promote the preferred nucleation and rapid growth of vertically aligned ZIF-L nanocrystals to construct dense membranes. Vertically aligned nanochannels between the interlattice of ZIF-L are therefore formed through the polycrystalline membrane. The membrane exhibited a high H2 permeance, 1635 GPU (1 GPU = 1 × 10–6 cm3 (STP)/(cm2 s cmHg)), and H2/C3H8 selectivity of 516, when targeting hydrogen separation from hydrocarbon in a steam reforming process. The membrane can be further used in organic solvent nanofiltrations, with a methanol permeance of 38.7 L m–2 h–1 bar–1 and >90% rejection of organic dye molecules. Furthermore, by taking advantage of the anisotropic pore structure, the ZIF-L membrane could be further hydrolyzed to produce ultrathin ZIF-L nanosheets with a thickness ∼5 nm, which provides a facile platform to synthesize two-dimensional MOFs nanosheets.