Well-defined orthorhombic LiMnO2 was synthesized using LiOH and gamma-MnOOH as starting materials at 1000degreesC in an argon atmosphere by quenching method. It showed a very small initial discharge capacity of about 34 mAh/g at room temperature. However, after a 24 h grinding, the LiMnO2 delivered not only a large initial discharge capacity of 210 mAh/g, but also a high cycling retention rate of 96% in the (3 + 4) V region after 50 cycles. We found that the grinding treatment was very effective to activate lithium insertion/extraction into the LiMnO2 layers in the 3 V region.