We synthesized micrometer-sized LiCoMnO4 with spinel structure by heating ion-exchanged LixCo0.5Mn0.5O2 with O6-type layered rocksalt structure at 600 degrees C. In this synthetic route, we first synthesized micrometer-sized NaxCo0.5Mn0.5O2 with P2-type layered rocksalt structure at 900 degrees C. Then, we prepared O6-LixCo0.5Mn0.5O2 by ion-exchange of Na+ for Li+ using LiNO3 at 280 degrees C. The structural transformation from layered rocksalt to spinel structure was confirmed at about 600 degrees C by the DTA curve. The average particle size of about 5 mu m was almost the same as that of the starting P2-NaxCo0.5Mn0.5O2. The discharge capacity-voltage curves of the present LiCoMnO4 showed a maximum discharge capacity of about 105 mA h g(-1) between 3.0 and 5.2 V vs. Li/Li+ at 25 degrees C, in spite of large micrometer-sized particles. We believe that micrometer-sized LiCoMnO4 would be important as one of 5 V cathode materials to achieve its higher compressed density of the electrode for advanced lithium ion batteries (LIBs), such as all solid-state LIB application.