The present work studied the evolution of stresses and associated mechanisms in the zirconia layer formed during the oxidation of Zircaloy-4 under a He/O2 mixture, at temperatures of 700°C, 800°C and 900°C. Measurements by X-ray diffraction are performed in-situ under synchrotron radiation during oxidation to determine the evolutions of phases and stresses in the oxide layer with time. The results show that the zirconia formed contains a mixture of monoclinic and tetragonal phases. The proportion of the tetragonal phase depends on the oxidation temperature and decreases during oxidation. In order to better understand the influence of this evolution and localize the tetragonal phase, measurements by Raman spectroscopy have also been performed. These two phases are subjected to compressive stresses in directions perpendicular to the oxide layer growth direction. These stresses depend on temperature and relax during oxidation. Two mechanical models either considering the zirconia phases independently or considering an equivalent homogeneous oxide are proposed to describe the evolution of these stresses, considering that it is due to oxide viscoplasticity. The model parameters are analysed to discuss the mechanisms of viscoplastic flow in the oxide. Numerical values for the viscoplastic parameters of the model as well as for the corresponding activation energy are therefore provided.