Thermal properties and crystallization behav-ior of PET were investigated by controlling the annealing conditions, such as relative humidity, annealing temperature, and annealing time. The variations of moisture content, glass transition temperature (T g ) and cold crystallization temperature (T cc ) were analyzed after annealing the PET specimens. Crystallization processing was performed with annealed PET specimens, and then the degree of crystallinity and the heat distortion temperature (HDT) were examined. Moisture content in the PET specimens increased with increasing the relative humidity, annealing temperature, and annealing time. T g and T cc of the annealed PET specimens decreased with increasing moisture content. The degree of crystallinity increased as increasing moisture content in the PET specimens. Although the moisture contents in the specimens after annealing were the same, the degree of crystallinity varied. The degree of crystallinity was affected by residual stress for constant moisture content. The relaxation of residual stress in the PET specimens differed depending on the annealing conditions, and the maximum degree of crystallinity increased with decreasing residual stress in the PET specimens.