In order to clarify the structure formation processes in the induction period of polymer crystallization the annealing time dependence of depolarized light-scattering (DPLS) intensities has been investigated as a function of crystallization temperature for poly(ethylene terephthalate), poly(ethylene naphthalate), syndiotactic polystyrene, and isotactic polystyrene. It is found that the induction period may be separated into three stages: the first stage where the DPLS intensity hardly changes with time, the second stage where the intensity increases exponentially, and the third stage where it levels off. Considering that the DPLS provides information about the degree of parallel orientation of rigid polymer segments, the first stage whose time length depends on the annealing temperature may be assigned to a process where the polymer chains begin to partially assume a rigid conformation, generally a helical structure being almost the same as the structure in the corresponding crystal. This process is limited to a time when the average length of the rigid segments attains a critical value given by a Shimada, Doi, and Okano theory [J. Chem. Phys. 88, 7181 (1988)] above which spinodal decomposition (SD) is caused. The second and third stages correspond to the early and late stages of SD, respectively, which was confirmed by small-angle x-ray scattering measurements. The apparent activation energies obtained from the temperature dependence of the DPLS intensities for the three stages were 35–40, 25–50, and 180–400 kJ/mol, respectively, for all the polymers. The large apparent activation energies for the late stage of SD is discussed within a framework of Binder and Stauffer’s theory [Phys. Rev. Lett. 33, 1006 (1974)]. © 2002 The American Physical Society.