Theoretical analysis of flexible-chain polymer crystallization was carried out over a wide temperature range from glass transition temperature Tgto melting temperature Tm. Temperature dependence of dynamic behavior manifesting in the decrease of crystallizing length with decreasing temperature was taken into account. The dependence of crystallizing length on temperature was obtained using chosen values of it at Tgand Tm. The crystallization with the formation of folded-chain crystals (type I) and uncoiled-chain crystals (type II) was considered. The analysis of thermodynamical favorability of both types of crystals with respect to temperature made it possible to obtain a flexible-chain crystallizing polymer phase diagram. This diagram shows the existence of two ranges where type II crystal formation is more favorable: a narrow range near Tmand the wider one near Tg, separated by the temperature range of crystallization with chain folding. Temperature dependences of type II crystals fraction in the system and their size were calculated. It is shown that the crystallization at considerable supercooling leads to the appearance of a great number of type II small crystals connected by tie chains. The system formed is characterized by a high degree of crystallite interconnection.