LiCoO2 single-crystal particles were obtained by supercritical water synthesis (SCWS) under optimized preparation conditions. The reaction time was less than 1 min. The LiCoO2 particle size was <1 mu m, which was much smaller than that of LiCoO2 prepared by a solid-state reaction, and the LiCoO2 crystal shape was rhombohedral. An electron diffraction image showed a highly crystalline LiCoO2 phase. Intercalation and deintercalation of lithium into and from LiCoO2 particles were carried out with good reversibility.