The cathode material Li x CoO 2 was synthesized by preliminary mechanochemical activation of precursor oxide mixture powders, followed by thermal treatment at 800 o C for 5h. The effects of the molar ratio of Li/Co on the electrochemical behavior were examined. The Li x CoO 2 at Li/Co=1.07 showed superior cycling stability to the Li/Co=1.0 sample. This is attributed to the disappearance of a phase transition related to monoclinic distortion and the relatively lowered transport resistance in Li/Co=1.07 sample.