Abstract: The formation of La₂O₃ nanoparticles in the La₂O₃-Mo cathode has been discussed by using XRD and SEM methods. In the reduction of MoO₂ powder doped with La(NO₃)₃, LaO₃ and MoO₂ may form solid solution and the following precipitation of La₂O₃ from Mo lattice will take place, which results in the formation of La₂O₃ nanoparticles. The sizes of La₂O₃ particles in nanometer and micrometer in the La₂O₃-Mo body depend on the distribution of La₂O₃ nanoparticles on the surface of Mo particles. Another explanation of La₂O₃ nanoparticles in the La₂O₃-Mo cathode is as follows. The powdered La₂O₃ nanoparticles will melt during the high temperature sintering of La₂O₃-Mo materials. The La³⁺ and O^2- produced by the break-off of La-O bonds may substitute for Mo atoms and precipitate from Mo lattice, then recombine into La₂O₃ nanoparticles.