Ultrafine powders of LiCoO2 were prepared under mild hydrothermal conditions. Reactant mixtures of aqueous solutions of cobalt (II) nitrate, lithium hydroxide and hydrogen peroxide with different Co/Li and Co/H2O2 ratios were hydrothermally treated at 150-250 degreesC for 0.5-24 h in a Teflon-lined autoclave. Only HT-LiCoO2 phase. was observed by X-ray diffraction (XRD) analysis. Scanning (SEM) and transmission (TEM) electron microscopies revealed the formation of well-crystallized hexagonal platelike particles with average size 70-200 nm. The increase in LiOH concentration results in better crystallinity. Annealing of hydrothermally prepared LiCoO2 at 230 degreesC leads to decreasing initial capacity from 130 down to 120 mA h/g, but improves cyclability (fade rate drops from 3.1 to 1.6 mA h/g per cycle). Submicrometric particle size and high surface area results in good electrochemical properties for high discharge rate.