Co-doped orthorhombic LiMnO2 powders were synthesized by the hydrothermal treatment of tetragonal (CoxMn1-x)(3)O-4 as a precursor with aqueous lithium hydroxide solution at 170degreesC. The X-ray diffraction and chemical analyses of the hydrothermal products confirmed a well-ordered stoichiometric orthorhombic structure (LiCoxMn1-xO2, Pmnm) with the wide substitution range of 0 less than or equal to x less than or equal to 0.14. The prepared powders were single-crystallite particle oxides, and the layer structure of about 5.7 Angstrom spacing for LiCo0.14Mn0.86O2 was directly observed by high-resolution transmission electron microscopy. The Co doping of x = 0.1 in LiCoxMn1-xO2 was effective in improving the cycling performance as a Li-ion battery cathode substantially. The Co substitution in Mn sites of o-LiMnO2 was not only to improve the structural integrity, but also to enhance high capacity retention upon extensive cycling. This new hydrothermal synthesis is expected to be an excellent alternative powder preparation method of high-capacity cathode materials to be used for Li-ion secondary batteries.