The molecular design of nanoporous membranes with desired morphology and selectivity has attracted significant interest over the past few decades. A major problem in their applications is the trade-off between sieving property and permeability. Here, we report the discovery of elongation-induced nano-pore evolution during the external stretching of a novel polyamide-imide nanofiltration hollow fiber membrane in a dry-jet wet-spinning process that simultaneously leads to a decreased pore size hut increased pure water permeability. The molecular weight cutoff, pore size, and pore size distribution were finely tuned using this approach. AFM and polarized FTIR verified the nano-pore morphological evolution and an enhanced molecular orientation in the surface skin layer. The resultant nanofiltration membranes exhibit highly effective fractionation of the monovalent and divalent ions of NaCl/Na2SO4 binary salt solutions. More than 99.5% glutathione can be rejected by the nanofiltration membranes at neutral pH, offering the feasibility of recovering this tripeptide. (C) 2009 American Institute of Chemical Engineers AIChEJ, 56: 1481-1494, 2010