Layer-structured Li-Ni-based oxide has been studied extensively as the promising cathode material for lithium ion battery because of its lower cost and higher practice capacity compared with the currently used LiCoO2. In this study, the wide solid solutions of Mg-doped LiNi0.6−y Mg y Co0.25Mn0.15O2 (0≦ y ≦0.08) with the layered α-NaFeO2 structure were synthesized by the mixing hydroxide method. The capacity retention of LiNi0.57Mg0.03Co0.25Mn0.15O2 in both room temperature and 55°C was increased to 93%. The oxidation state and local environment of transition metals (Ni, Co and Mn) were studied by the high energy synchrotron X-ray absorption spectroscopy (XAS). The X-ray absorption near edge structure (XANES) indicated that the initial valences were 2+/3+, 3+ and 4+ for Ni, Co, and Mn, respectively, in the pristine LiNi0.57Mg0.03Co0.25Mn0.15O2. The in situ XAS study with the cell charge to 5.2V showed that the main redox reaction during delithiation was achieved by Ni (i.e. Ni2+/Ni3+ →Ni4+). The EXAFS data also exhibited that the bond length of NiO decreased drastically, while the CoO and MnO distances exhibited a slight change with the decrease of Li content in the electrode. It was further revealed that all the second shell metal–metal (NiM, CoM, and MnO) distances decreased due to the oxidation of metal ions and the contraction in a-axes of this structure.