Fully oriented, partially oriented, drawn, and quenched samples of poly (ethylene terephthalate) (PET) filaments were examined. Crystalline fraction and the average size of crystallites were measured by a wide angle X-ray scattering diffractometer. Sample geometry in X-ray diffraction was found to affect the intensity of the diffraction patterns. The crystalline fraction of the samples can be measured by X-ray diffraction in a symmetrical reflection geometry accompanied with curve fitting computer software. Annealing improves the average size of crystallites in a direction perpendicular to (010) plane more than the two other directions, namely perpendicular to (100) and (-1.05) planes. Fourier transform infrared spectroscopy was used to determine the percentage of trans configuration, and the volume crystalline fraction was determined from density measurements. A good correlation was obtained between the percentage of trans configuration and weight percentage of crystalline fraction determined by density measurements. The molecular orientation of the samples was measured by a polarizing microscope and its results conform to the other findings. The presence of extended chains in the noncrystalline regions supports the idea of the presence of oriented amorphous (amorphous with correlation) and nonoriented amorphous domains in PET drawn filaments. (c) 2007 Wiley Periodicals, Inc.