X—ray diffraction as well as the transmission technigue have been used to studynine types of poly (ethylene terephthalate) industrial filaments made in various domesticand overseas factories．X—ray diffraction patterns of crystal planes 010

110

100 in equa-tor line and 105 in meridian line ate measured．By means of peak—separating techniquethe average cross-sectional area

the average length and the average volume of the crystalgrains are calculated．The result shows the obvious differences among the various filaments．Primary knowledge

average crystallite size was concerned with shrinkage of fibers.