A low-temperature, hydrothermal process for the synthesis of high-quality intercalation oxide LiMnO2 has been developed. Mn3O4 as a hydrothermal precursor was prepared by oxidation of Mn(OH)(2) by bubbling O-2 at 80 degreesC. The fine single crystalline particle oxide, Mn3O4, was hydrothermally treated with various LiOH concentrations at 170 degreesC. The mechanism of phase evolution has been studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Rietveld analysis revealed that a well-ordered orthorhombic LiMnO2 having a zigzag layer structure (Pmnm) was readily formed by hydrothermal reaction. The product is electrochemically active and shows relatively high capacity upon cycling.