The method of variance-range function[1] was used for the analysis of X-ray line proflie of 010 and 100 diffraction of annealed PET fibers．Their average crystallite sizes M and the distortion factors g of second kind lattice diatortion were determined．The plane-spa-cings d of the (010) and (100) crystal planes were also obtained from the peak separation procedures. In M as afunction of l/Ta (Tai annealing temperature) are reported in Fig．2．h is shown that the fitting of each set of data needs two straight lines，crossing each．other at T*= 503 K．The polydispersity indices of crystallite size Mm／M are shown in Fig．3 as a function of n．There is a maximum in each of the Mm／M～Ta curves at T*=503 K． It can be seen from above mentioned observation that densitification and growth of crystalites in PET fibers take place following different molecular mechanism at temperature above or below the T*．The molecular motion of smaller unit can take place belOW the T*．The cooperation motion of larger unit (=3.5—4 smaller units) is allowed just above the T*．