The purpose of this paper is to show the possibility for optimizing the post growth annealing process of CeO2 thin films sputtered on (111) Si by using in situ Raman characterization. In fact, the post growth annealing is an important step in the microelectronic process and consequently the studies, which permit to find the good annealing conditions, are time consuming because many annealing temperatures, many annealing times and many annealing atmospheres should be tested and characterized for each test. For these reasons, we propose to characterize the crystallinity of CeO2 thin films by in situ Raman spectroscopy during the post growth annealing. This original method permits to observe, in operando, the evolution of CeO2 thin film crystallinity versus the annealing temperature and/or the annealing time changes. In fact, the lowest Raman line broadening, which is related to an improvement in crystallinity of the CeO2 layer, has been obtained with an annealing performed at 900°C for 3h. So, the determination of the best annealing parameters is much faster with this method than with classical methods. To our knowledge, it is the first time that the optimization of post-annealing parameters by using in situ Raman characterization has been reported for thin films.