Crystallization of polymers was studied in terms of density fluctuation of amorphous phase by using amorphous poly(ethylene terephthalate) (PET) films in disoriented and oriented states. When the amorphous film was put into a hot oven with desired temperature and an incident beam of He-Ne gas laser was directed to the film, the scattered intensity against time was classified into three regions. In the first stage, the scattered intensity hardly changed. In the second stage, the logarithm of scattered intensity increased linearly. This tendency was apparently in accordance with concentration fluctuation of linear theories of spinodal decomposition (SD) originally proposed by Cahn for small molecules and that modified by de Gennes for the polymer system which describe the initial stage in the phase separation process. In the third stage, the increase in intensity started to deviate from the linear relationship and tended to level off. The scattering maximum increased with time and maintained at the same scattered vector in stage II and then tended to shift toward a lower angle in stage III, characterizing SD. Surprisingly, the above phenomenon indicates that the observation scale of the density fluctuation associated with the driving force of the crystallization process was several thousand nanometers. Such behavior was quite different from the density fluctuation at smaller scales detected by X-ray diffraction and scattering. On the basis of the second stage, the apparent spinodal temperatures (T-s) could be estimated from the dynamics measured as a function of temperature for undrawn and drawn PET films. The growth rate maximum of the density fluctuation was independent of the scattered vector q and increased with annealing temperature. This indicates that the phase separation due to density fluctuation occurs in thermodynamic unstable state and crystallization takes place partially in the higher density region of amorphous chains. This mechanism is different from crystallization associated with nucleation and growth. To demonstrate this concept in detail, further analysis was done by using wide-angle X-ray diffraction, differential scanning calorimetry, and small-angle light scattering under polarization conditions and density measurement.