As the cathode materials for rechargeable lithium batteries, five commercial lithium cobalt oxide powders have been investigated for a comparative study. The X-ray diffraction analysis indicates that all these powders exhibit the α-NaFeO2 layered structure. The size distribution and morphology were analyzed by particle sedimentation method and scanning electron microscopy (SEM). Their electrochemical properties including cycleability and especially 3.6V plateau efficiency, a recently required control parameter, are compared. Two kinds of modifications, i.e. Li2CO3 coating and high-temperature treatment, have been applied to improve the electrochemical performance of one of these five powders. After the high-temperature treatment in air, cobalt oxidation-state becomes higher and Li(Li x Co1−x )O2 is formed. Both of the two modified means can significantly improve the 3.6V-plateau efficiency through suppressing the cell impedance rise during cycling. A general discussion on the factors influencing the plateau efficiency is also given.