High-Ni-content layer-structured cathode materials for lithium-ion secondary batteries have high capacity but they suffer from poor thermal stability. We studied the mechanism responsible for their thermal stability to make them more stable. We used X-ray absorption fine structure (XAFS) spectra to clarify the changes in the oxidation states and the local structures for each transition metal in Li0.2Ni x Mn(1−x)/2Co(1−x)/2O2 (x = 1/3, 0.6, and 0.8) at high temperature. The X-ray absorption near edge structure (XANES) spectra indicated that the oxidation state of Ni and Co changed due to heating. Although, pre-edge of XANES spectra indicated that the occupation sites of Co ions changed from octahedral to tetrahedral, Mn ions remained in the octahedral sites at high temperature. The extended X-ray absorption fine structure (EXAFS) results supported the change in the occupation sites of Co and Mn ions due to heating. It can be concluded that Co and Mn affected thermal stability of those Ni-based cathode materials differently. The Co ions migrated from octahedral to tetrahedral sites, and they stably occupy tetrahedral sites. Hence, so the structural change from spinel to rock-salt is suppressed. The oxidation state of Mn is stable and remains unchanged in layer structure at high temperature.