Microstructural changes in LiNi0.5Co0.2Mn0.3O2 cathode material after charge/discharge cycles were investigated by transmission electron microscopy and electron energy loss-spectroscopy. The microstructure on the surface of the cycled primary particles of LiNi0.5Co0.2Mn0.3O2 changed from an ordered rock-salt structure into a metal monoxide-type rock-salt structure, and this was accompanied by reduction of the oxidation states of the transition metal ions, especially the Mn4+ ions. It should be noted that the (0001) surface of the primary particles remained intact after the cycling test. These results indicate that the degradation mechanism of LiNi0.5Co0.2Mn0.3O2 is different from that of other LiNiO2-based cathode materials previously reported. Tailoring the transition metal ion content ratios is expected to give LiNi1−x−y Co x Mn y O2 various levels of surface stability.