LiNi1/3Co1/3Mn1/3O2 and LiCoO2 cathode materials were synthesized by using a supercritical water (SCW) method with a metal salt solution in a batch reactor. Stoichiometric LiNi1/3Co1/3Mn1/3O2 was successfully synthesized in a 10-min reaction without calcination, while overlithiated LiCoO2 (Li1.15CoO2) was synthesized using the batch SCW method. The physical properties and electrochemical performances of LiNi1/3Co1/3Mn1/3O2 were compared to those of Li1.15CoO2 by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and charge/discharge cycling tests. The XRD pattern of LiNi1/3Co1/3Mn1/3O2 was found to be similar to that of Li1.15CoO2, showing clear splitting of the (006)/(102) and (108)/(110) peak pairs as particular characteristics of the layered structure. In addition, both cathode powders showed good crystallinity and phase purity, even though a short reaction time without calcination was applied to the SCW method. The initial specific discharge capacities of the Li1.15CoO2 and LiNi1/3Co1/3Mn1/3O2 powders at a current density of 0.24mA/cm2 in 2.5–4.5V were 149 and 180mAh/g, and their irreversible capacity loss was 20 and 17mAh/g, respectively. The discharge capacities of the Li1.15CoO2 and LiNi1/3Co1/3Mn1/3O2 powders decreased with cycling and remained at 108 and 154mAh/g after 30 cycles, which are 79% and 89% of the initial capacities. Compared to the overlithiated LiCoO2 cathode powders, the LiNi1/3Co1/3Mn1/3O2 cathode powders synthesized by SCW method had better electrochemical performances.