The most highlighted point of this work to emphasize is that it is the first trial to use Mn3O4 oxide as a precursor to synthesize orthorhombic LiMnO2 by the hydrothermal method. A well-ordered orthorhombic LiMnO2 phase was formed by the hydrothermal treatment of Mn3O4 with excess LiOH aqueous solution at 170°C. According to TEM observation, the as-synthesized powder was single crystalline particle oxide. Comparing with other orthorhombic LiMnO2 prepared by low temperature synthetic route and by high temperature calcination, the orthorhombic LiMnO2 prepared by the hydrothermal route showed enhanced battery performance as a lithium battery cathode material. We believe that the new hydrothermal synthesis is expected as an excellent alternative of powder preparation method of high capacity cathode material to be used for Li-ion secondary battery.