Well-defined orthorhombic LiMnO2 was synthesized using LiOH and γ-MnOOH starting materials at 1000°C in an argon flow by quenching process. X-ray diffraction (XRD) revealed that the compound showed an orthorhombic phase of a space group with Pmnm (a=2.806 Å, b=5.750 Å, and c=4.593 Å). The prepared compound was composed of particles of about 5–15 μm diameter with a bar-shape and small spherical one of about 1–2 μm. It showed very small initial discharge capacity of about 34 mA h g−1 in the (3+4) V region at room temperature. However, after 12 h grinding, the LiMnO2 delivered 201 mA h g−1 in the first cycle and still delivered 200 mA h g−1 after 50 cycles at room temperature. We found that the initial discharge capacity of LiMnO2 agreed well with its specific surface area by Brunauer, Emmett and Teller (BET) analysis. Especially, the grinding treatment played an important role to activate the lithium insertion–extraction into the LiMnO2 layer in the 3 V region.