Solid state polymerization of poly(ethylene terephthalate) (PET) has been carried out under reduced pressure of 0.2 torr at the various temperatures ranging from 200 to 23$0^{circ}C$. The increase in the molecular weight of resulting polymer has been observed, i.e., the higher molecular weight of the resulting polymer with higher temperature and longer reaction time. It has been found that the linear dependence of the rate of molecular weight increase on reaction time in the early stage of the reaction is no longer valid when the number average molecular weight of the polymer reaches value of 30, 000. The change in the crystal structure and crystallization process has been interpreted by means of density measurement, X-ray diffraction and DSC.