A mild hydrothermal method using Li-birnessite (LixMnO2·nH2O) ultrafine fiber as the precursor has been adopted to prepare Li4Mn5O12, which is of interest as an electrode material for 3 V rechargeable lithium ion batteries. X-ray diffraction data reveal that the obtained powders have a pure spinel structure with a lattice constant of 8.135 Å. The scanning electron microscopy image of the obtained powders shows the particles are cubic-shaped whose average size is about 40–50 nm. The results from inductively coupled plasma-atomic emission spectroscope and wet chemical analysis indicate that a Li/Mn ratio of 0.796, and an average valence of 3.96 of Mn ion have been achieved in the as-prepared products. The thermogravimetric and differential thermal analysis data also agree with the previous reports on Li4Mn5O12, suggesting that near stoichiometry of Li4Mn5O12 has been synthesized by this procedure at the rather low temperature 110°C.