Abstract: Cerium (Ce) is a highly significant rare earth element with diverse applications in magnets, phosphors, alloys, catalysis, and batteries. Currently, investigations into the oxidation of metallic cerium predominantly concentrate on environments involving oxygen, air, and humid air. In contrast, there is a scarcity of research pertaining to steam atmospheres, and even fewer studies have examined the microscopic mechanisms underlying steam oxidation. The investigation of the microscopic processes and mechanisms involved in the steam oxidation of Ce is essential for gaining a deeper understanding of its physical and chemical properties, which is crucial for enhancing its commercial applications. Furthermore, Ce serves as an exemplary metal simulation material within the domain of nuclear engineering, and the exploration of its steam oxidation holds considerable importance for examining the conversion behaviors of significant actinide metals, including uranium and plutonium. To investigate the microscopic processes underlying the oxidation of Ce in water vapor, this study developed an experimental apparatus utilizing in situ X-ray diffraction (XRD) technology, which enables real-time monitoring of the phase structure evolution during the oxidation process. To ascertain the structure, composition, and morphology of the oxidation products of Ce in water vapor, a combination of XRD, Raman spectroscopy, and transmission electron microscopy (TEM) techniques was employed for analysis and characterization. Furthermore, to investigate the mechanism of Ce steam oxidation, the Vienna Ab initio Simulation Package (VASP) was employed to optimize the crystal structure. The Perdew-Burke-Ernzerhof (PBE) functional was utilized for the calculations, and the continuous image nudged elastic band (CINEB) method was applied to identify the transition state associated with the oxidation process of Ce in water vapor. The findings indicate that CeH_x serves as an intermediate product in the water vapor oxidation of Ce, with H₂O exhibiting a greater propensity for adsorption and dissociation on its surface. The generation of CeH_x contributes to a reduction in the activation energy required for water vapor oxidation, thereby significantly influencing the kinetics of this process. The primary product resulting from steam oxidation is cerium dioxide (CeO₂), within which edge dislocation defects is identified. These linear defects alter the diffusion pathways of hydroxide (OH⁻) and oxide (O²⁻) anions in CeO₂, potentially enhancing the rate of steam oxidation. It is concluded that the generation and release of stress within the oxide film are critical factors in the fracture and spallation of the CeO₂ film, as well as in the development of linear crystal defects. This study elucidates the mechanism underlying the oxidation of cerium in water vapor, highlighting the continuous oxidation and formation of CeH_x at the oxide-metal interface. The theoretical analysis suggests that the dehydrogenation process of CeH_x is the only step requiring heat absorption, which is identified as the rate-controlling step in the steam oxidation of cerium.