Electronic structures of Li(Co, Al)O2 and Mg-doped Li(Co, Al)O2 were studied theoretically via ab initio calculation based on density-functional theory. Studies of band structures and density of states show that hole states appear in the valence bands of the Mg-doped material as well as an increase in bandwidth, which are in charge of the increase in the observed electronic conductivity. It was found by analysis of Co3d density of states that the divalence Mg doping induces a higher cobalt valence state between Co3+ and Co4+ in Li(Co, Al, Mg)O2, which is in good agreement with experimental results. Using the band theory, we have calculated the average valence states of Co and O.