Using Mn(OH)2 as precursor, LiOH as lithiating agent and (NH4)2S2O8 as oxidant, layered o-LiMnO2 was obtained by a novel method--in situ oxidation-intercalation under mild conditions (80℃). The product was characterized by XRD, ICP, TEM and 7Li-NMR. The results reveal that orthorhombic LiMnO2 with high purity and good crystallinity can be obtained by this method. During electrochemical tests, a LiMnO2/Li cell shows an initial reversible capacity of 208 mAh·g-1 and a reversible capacity of 180 mAh·g-1 after 30 cycles at room temperature.