The aim of this study is the determination of the residual stress state and the analysis of the microstructure of ZrO(2) oxide layers (mainly the tetragonal to monoclinic phase ratio) on pre-oxidized samples at 400degreesC and 20degreesC using x-ray diffraction techniques. The layer was formed on two zirconium alloys plates (Zy-4 and Zr-1%Nb-O) oxidized in a static autoclave with PWR chemistry at 360degreesC for several lengths of time. The marked texture and the large stress and phase distribution gradients in the zirconia layer did not allow to use a conventional x-ray apparatus, especially for local and in-situ analyses. These difficulties have been overcame using the French Synchrotron Radiation (SR) facility LURE (Laboratoire pour l'Utilisation du Rayonnement Electromagnetique) at Orsay, France. The fraction of tetragonal zirconia at different depths in the oxide layer and the level of residual stresses in the monoclinic phase of the oxide scales are determined using the synchrotron H10 beam line both after cooling and in-situ at 400degreesC. In fact, stresses determined at room temperature are certainly not representative of the ones spreading out during the growth of the oxide scale because of the anisotropy of thermal expansion coefficients, elastic constants and texture of Zr and ZrO(2). Therefore, we use a thermoelastic self-consistent model to determine the local stresses developing in the monoclinic oxide for particular crystallographic orientation (especially epitaxial and main orientations). The experimental data are compared with the results obtained from the model and also with a macroscopic calculation of the thermal stress in the oxide layer.