The phase-change characteristics of Ge2Sb2Te5/SiO2 multilayered films with various bilayer thicknesses were investigated using high-resolution transmission electron microscopy. The change in the electrical sheet resistance of the films shows that the metastable face-centered cubic (fcc) structure formation was significantly affected by the bilayer thickness of the multilayer film. That is, compared with the single-layered Ge2Sb2Te5 film, the multilayered films transformed to hexagonal at lower temperatures. In particular, the transition temperature region of the fcc structure with semiconductor properties was significantly reduced. We observed that as the bilayer thickness decreased and annealing temperature increased, the structure of the multilayer film was transformed into Ge-deficient and hexagonal phases such as GeSb2Te4 and Sb2Te3. This behavior was induced by the out-diffusion of Ge atoms from the outer surface layer, which can be caused by a difference in thermal strain at the interface between the Ge2Sb2Te5 and SiO2 films.