The presence of intermediate phase in the semicrystalline PET was suggested from the results of factor analysis and digital subtraction of IR spectra of PET samples annealed under various thermal conditions. The characteristic peak of intermediate phase was obtained around 1021cm-1, The peaks of pure amorphous and crystalline regions were found at 1017cm-1 and 1024cm-1, respectively. Thus the absorbance peak around 1020cm-1 was deconvoluted into three component peaks to investigate the change of each phase as functions of annealing temperature and time. In the intial crystallization step at low annealing temperature, the transformation of amorphous part into intermediate part was preferred to that of amorphous phase into crystalline one. On the other hand, intermediate phase as well as amorphous phase was tranformed into crystalline one in longer annealing time or at higher temperature. The densities of pure amorphous, intermediate, and crystalline phases were calculated as 1.2972 $pm$0.0101, 1.3851 $pm$0.0283, and 1.5036$pm$0.0240g/㎤, respectively, from the correlation of band deconvolution and density data.