Abstract: The efficient conversion of metals into nitrides and oxides is one of the key steps in nuclear fuel processing. In this work, metallic cerium(Ce) was used as a surrogate material. The nitridation reaction of metallic Ce was achieved in a mixed atmosphere of low-concentration water vapor(water vapor partial pressure ≤ 5.0 kPa) and nitrogen(N₂). The effects of water vapor partial pressure(0.5, 1.0, 2.5 and 5.0 kPa) and reaction temperature(310, 330, 350 and 370 ℃) on the nitridation reaction rate of metallic Ce were investigated. According to the pressure change during the reaction process and the characterization results of X-ray diffraction(XRD), it is found that metallic Ce reacts with N₂ in the presence of low-concentration water vapor and is completely converted to cerium nitride(CeN). Increasing the partial pressure of water vapor and the reaction temperature can accelerate the nitridation reaction rate of metallic Ce. When liquid water is used instead of water vapor, metallic Ce can still undergo the nitridation reaction and be converted into CeN, further simplifying the nitridation reaction process. In addition, through the characterization by XRD, Raman spectroscopy and X-ray photoelectron spectroscopy, it is demonstrated that the CeN product is rapidly oxidized and converted into high-purity cerium dioxide(CeO₂) in an air atmosphere. This work provides new insights into the mechanisms and processes of metal nitridation-oxidation reactions.