Many semiconductor devices, power microelectromechanical systems and complex nano-structures like multilayers use plasma-enhanced chemical vapor deposited (PECVD) oxide films as a main component. A mechanical stress - always present in thin films - can cause important changes in the work of the devices or can even lead to a damage of the devices. Therefore, there is an increasing need to understand and tailor processes to achieve process compatibility and device reliability. In this study, we present mechanical stress evolution of PECVD oxide films during and after postdeposition thermal cycling. PECVD oxide films 2.0 and 5.0 μm thick were deposited on 400 μm thick silicon wafers in a plasma CVD reactor at 300°C. Average mechanical stresses in the films were deduced from the warpage induced in the wafer due to films deposition by using the Stoney’s equation [1]. The wafers’ warpage was investigated with the help of a laser interferometer. Precision of the interferometer was 0.5 μm. The interferometer was equipped by special high-temperature external thermo-chamber. Thermal cycling of a variety of films with a heating rate of 50C/min was conducted and in situ wafer curvature was measured between room temperature and 5000C. After film deposition the wafers obtain a spherical form of deformation. This means that the distribution of mechanical stresses in PECVD oxide films is isotropic. The overall mechanical stress state in as-deposited film (room temperature) is compressive and determined by the superposition of two primary effects. The intrinsic (athermal) stress is generated during the deposition and is strongly related to the process parameters. The thermal stress only results from the temperature change between deposition and characterization and the difference in thermal expansion coefficients of the film and the wafer. It is found that the PECVD oxide film stresses strongly depend on the processing history and thermal stresses annihilation are identified as the major mechanisms to control the mechanical behaviour of the oxide films. The dependence of residual stress on temperature is non-linear with significant hysteresis. This hysteresis reduced significantly during the subsequent thermal cycle. Thermal cycling is shown to result in major plastic deformation of the film and a switch from a compressive to a tensile state of stress; both athermal and thermal components of the net stress alter in different ways during cycling. It should be emphasized that independent from the conditions of depositing films and subsequent thermal cycling after the chemical removal of PECVD oxide films, the wafers return to the initial non deformed state. This result shows the elastic nature of warpage.