The development of the changes of structure during thermal treatment of polyester fiber is studied. Critical dissolution time (CDT) was evaluated to trace the changes in crystallinity and crystal morphology of polyester fibers. CDT increased as the time of thermal treatment was increased. The change in CDT was less effective for samples thermally treated at temperatures up to 160 degrees C, which may indicate a nucleation tendency of tiny crystals. CDT of polyester treated at 200 degrees C in air was found to be relatively high, while silicon oil medium can affect both amorphous and crystalline regions, increasing the structure disorder. The effect of treatment under constant length revealed smaller CDT values than those attained by samples treated in free conditions. An indication to crystal size increase was traced by a decrease in peak width in X-ray diffraction pattern. Thermal treatment increased the crystallinity of polyester fibers up to 160 degrees C. Dry air medium was more effective in increasing the disorder in amorphous regions than silicon oil medium.