The stress-strain and modulus-strain curves from the treatment of tensile test data are more complete and suitable for examining the mechanical properties of fibers than just the stress-strain curve. This article analyzes typical modulus-strain curves of poly(ethylene terephthalate) and polyamide fibers through the physical structure changes in these fibers during stretching and discusses how to enhance the modulus for industrial use. (C) 2004 Wiley Periodicals, Inc.