Abstract Formation of U2C3 by the reaction of UC2 with MgO has been studied by means of chemical and X-ray analyses at temperatures 1450–1700°C in vacuo. The reaction was found to proceed generating CO and Mg vapor and to produce U2C3. The decarburization is represented by 2UC2+MgO→U2C3+CO+Mg. UO2 and UC were formed as the intermediate compounds. The amount of UO2 was nearly equal to that formed by the reaction: UC2+4MgO=UO2+2CO+4Mg. The lattice parameter of U2C3 was 8.0890 Å. The lattice parameters of UC2 and UC varied with the reaction time. The main reaction sequence was suggested to be: 1⁄4UC2+MgO=1⁄4UO2+1⁄2CO+Mg 7⁄4UC2+1⁄4UO2=U2C3+1⁄2CO. The experimental results were in good agreement with those obtained by thermochemical calculation.