Orthorhombic LiMnO2 was synthesized using LiOH and gamma -MnOOH at 1000 degreesC in the argon flow by quenching method. Xray diffraction revealed that the LiMnO2 showed a well-defined orthorhombic phase of a space group with Pmnm. The lattice constants were a = 2.806 Angstrom, b = 5.750 Angstrom, and c = 4.593 Angstrom. The LiMnO2 after grinding delivered 212 mAh/g in the 9th cycle and still delivered 200 mAh/g after 50 cycles at room temperature, The well-defined orthorhombic LiMnO2 by the quenching method exhibited an excellent cycle performance.