In situ and ex situ X-ray diffraction are used to evidence the heterogeneous strain generation during the thermal crystallization of capped Ge2Sb2Te5 thin film. Upon crystallization, a considerable tensile stress builup linked to densification is observed and the strain (macroscopic and microscopic) increases as the crystallization advances. Two populations of oriented grains are distinguished. A depth analysis shows that the two populations of grains are a differently distributed in the depth of the film. Strain and structural gradient (or heterogeneity) is evidenced and shows that interfaces play a major role in the mechanical state of Ge2Sb2Te5 films. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3582355] All rights reserved.