The phase change from amorphous to crystalline state which occurs upon thermal annealing in the prototypical material Ge2Sb2Te5. Phase Change Materials (PCM) thin films is studied by concomitant, complementary and combined in-situ and X-ray diffraction (XRD) and X-ray reflectivity (XRR) techniques. Combined in-situ X-ray scattering techniques demonstrates the possibility to investigate with accuracy the structural, morphological and mechanical variations occurring in the films upon crystallization. The crystallization process is correlated to a volume shrinking (densification and thickness reduction) and to structural change with the building up of tensile strain. The combined XRD/XRR analysis gives new insights on the stress components built up in phase change material. Afterwards, concomitant grain growth, viscous flow, densification and thickness accommodation are observed which leads to a partial stress relaxation in the PCM films. This combined characterization techniques proposes a new approach in order to go further into the understanding of the involved phase change.