Ge–Sb–Te (GST)-based PCM alloys are currently used in optical data storage. The crystallization of GST materials is the rate-limiting step for these devices, hence a deeper knowledge of the crystallization mechanism is crucial for insightful development of faster devices. In the present work, the diffraction patterns for GST-225 thin films are studied using the in situ Grazing Incidence X-ray Diffraction method upon heating. It is shown that initial amorphous film in the temperature range from 120 to 140 °C is crystallized into two phases-cubic GST-225 (Fm3 ¯ m), and trigonal GST-147 (P3 ¯ m1). The crystallized film is stressed and highly textured, and should be characterized by the value of the lattice parameters averaged over all crystallographic planes. The structural transition of GST-225 from cubic to trigonal phase begins at T > 180 °C. The appearance of large-scale inhomogeneities in GST-225 film at T ≥ 100 °C indicates that the process of rearrangement of atoms Ge, Sb, and Te in the as-deposited amorphous film begins long before the onset of crystallization.