The electrochemical and charge/discharge behaviors of cobalt oxide prepared by the calcination of cobalt hydroxide were studied by the cyclic voltammetry (CV) and galvanostatical charge/discharge methods. The cyclic voltammograms of cobalt oxide containing 20% carbon black were correlated well with the results in the charge/discharge behaviors of Li/CoO batteries. A cathodic peak with potential of 0.5V (versus Li/Li+) in the first cycle of CV corresponded to the plateau with voltage of 1.0–0.8V in the first discharge cycle of Li/CoO battery was deduced to be the reduction and decomposition of CoO particle to be the nano-sized Co and Li2O. Both of the cathodic peak and the discharge plateau voltages were shifted to higher values in the following cycles due to the reduction of CoO particle from micro to nano-size.