In the paper, we have investigated the preparation and electrochemical performance of the solid solution Li2MnO3–LiCoO2 (or written as Li[Li x/3Co1−x Mn2x/3]O2). The chemical composition determined by ICP–AES indicates that the chemical formula of our prepared samples is very closely to that of the ideal Li2MnO3–LiCoO2 solid solution in chemical stoichiometry. It was found that the oxidation states of Co and Mn are +3 and +4, respectively by redox titration and X-ray photoelectron spectroscopy (XPS). XRD shows that most peaks of the samples prepared at 950°C could be simply indexed as a layered α-NaFeO2 structure (space group R 3 ¯ m , no. 166) except some additional peaks between 21° and 25°, which is caused by the appearance of the superlattice structure. The monoclinic phase has been formed for Li[Li x/3Co1−x Mn2x/3]O2 (x =0.5) at 750°C, which is reversibly transformed into a hexagonal phase with the increasing of sintering temperature from 750 to 950°C. It is found that the charge capacity of the samples Li[Li x/3Co1−x Mn2x/3]O2 (0.5≤ x ≤0.7) increases and the discharge capacity has no obvious change with the increasing of the sintering temperature due to the appearance of a flat charge voltage plateau at 4.45V. Moreover, the discharge capacity of the samples gradually reduces with the increasing Li2MnO3 content. The sample has the excellent electrochemical cycling performance with the charge–end voltage of 4.5V. XRD patterns of the charged product indicate that the layered structure has been kept during the charge process, which may be the reason for its good cycling performance.