Polycrystalline zeolitic imidazolate framework ZIF-8 membranes have shown great potential for energy-efficient propylene/propane separation. Their large-scale applications are, however, hampered by the lack of facile and cost-effective processing technologies for ultrathin ZIF-8 membranes on scalable substrates. Here, we report facile preparation of ultrathin ZIF-8 membranes on the bore side of Matrimid hollow fibers based on a polymer modification strategy. Site-selective nature of the method enables a positional control over locations (i.e., bore or shell side) of active ZIF-8 layers on the hollow fibers. The ZIF-8 membranes grown on the bore side of Matrimid hollow fibers with polydimethylsiloxane coating displayed an average propylene permeance and a separation factor of , similar to 152 X 10(-10) mol.m(-2).Pa-1.s(-1) and similar to 55, respectively. Optimization of ZIF-8 seeding step resulted in an improvement in propylene permeance (similar to 369 x 10(-10) mol.m(-2).Pa-1.s(-1)) while maintaining relatively high separation factor (similar to 40).