A series of LiNi0.5Mn0.5− x Co x O2 (0≤ x ≤0.5) compounds was prepared by a solid state reaction, and their structural, morphological and electrochemical characteristics were also investigated. Co doping suppresses the formation of the impurity phase, NiO, and improves the crystallinity of the compounds. Moreover, the unequilavent substitution of cobalt for manganese would give rise to the increase in the average valence of nickel, thereby shrinkage in the lattice volume and improvement of the electrochemical properties. LiNi0.5Mn0.2Co0.3O2 shows a high reversible capacity as well as an excellent rate capability in 3–4.6V.