The electrochemical behavior of LixCoO2 (x > 1.0) and its high pressure oxygen-treated samples were examined as cathode of lithium ion battery. LixCoO2 was prepared at nominal compositions of Li/Co = 1.02, 1.05, 1.10, 1.15, 1.20, 1.25, and 1.30. The practical compositions and cobalt valences were analyzed by ICP and iodometric titration technique. X-ray diffraction patterns show that the lattice gradually contracts with increasing Li/Co ratio. The lattice contraction stops at Li/Co = 1.15, which corresponds to the solid solution limit. The high oxygen pressure treatment reduced the number of oxygen defects and enlarged the interlayer distance. The electrochemical behavior of oxygen-treated samples show better cycling efficiency and less irreversible capacities. The reversible capacities show clear dependence on Li/Co ratio that the highest capacity of 140 mA h g(-1) was obtained when Li/Co = 1.10 and beyond that value it decreased linearly with Li/Co ratio.