Phase pure spinel LiMn2O4 nanoparticles can be directly synthesized by one-step hydrothermal reaction of γ-MnO2 with LiOH in an initial Li/Mn ratio of 1 at 200°C. The reaction might involve a redox reaction between Mn4+ and OH−, and the formation of LiMn2O4 at the same time under the proposed hydrothermal conditions. This hydrothermal process is simple since only γ-MnO2 powders are used as the Mn source, whereas without use of any oxidants, reductants, or low valence Mn source. The electrochemical performance of the as-synthesized LiMn2O4 nanoparticles towards Li+ insertion/extraction was examined. Rather good capacity and cycle performance, and an especially excellent high rate capability, were observed for the sample that was hydrothermally reacted for 3 days.