$LiCoO_2$, a cathode material for lithium rechargeable batteries, was prepared in a nanoscale through a simple sonochemistry. First, $Co_3O_4$ nanoparticles were prepared by reacting NaOH and $CoCl_2$ or $CoSO_4$ with a sonochemical method, operated at 20 kHz and 220 W for 20 min, very powerful multibubble sonoluminescence conditions for chemical reactions. Second, LiOH was coated onto the $Co_3O_4$ nanoparticles by the same method as above. Finally, $LiCoO_2$ nanoparticles of about 10~30 nm size in diameter were obtained by the thermal treatment of the resulting LiOH-coated $Co_3O_4$ nanoparticles at $500^{circ}C$ for 3 hr. This synthetic process is relatively quite mild and simple compared to the known method for the synthesis of $LiCoO_2$ nanoparticles. The materials synthesized were characterized by infrared spectroscopy, X-ray diffraction, inductively coupled plasma spectrometer, and high resolution-transmission electron microscopy analyses.