Abstract: The microstructure and crystal structure of oxide films in different depth of oxide thickness, and the morphology of fracture surface of oxide films, which are formed on Zircaloy-4 specimens during the autoulave tests in deionized water at 360 ℃/ 18.6MPa, have been investigated by transmission electron mi- croscopy, scanning electron microscopy and scanning probe microscopy. The results show that except the stable monoclinic crystal structure, the meta-stable phases including cubic, tetragonal and amorphous phases can be observed in the bottom layer of oxide films due to the presence of defects induced by the compressive stress in oxide crystals during the formation of zirconium oxide. Under the action of temperature, stress and time, the diffusion, annihilation and condensation of defects as vacancies and interstitials will be taken place in the middle and surface layer of oxide films. The inner stress of oxide crystals will be relaxed and the meta-stable phases will transform to the stable phase. Vacancies absorbed by grain boundaries form the clus- ters of pores in nanoscale, which will weaken the bonding force between grains. In this case, the grains will become gradually spherical due to the action of surface tension. Based on the formation of the clusters of pores and the development of cracks in oxide films,the phenomenon of corrosion transition will be induced due to the loss of protective characteristic of oxide films. This is an inevitable result caused by the micro- structure evolution of oxide films during the autoulave tests.