Abstract: The crystal structure change of Li_xCoO₂(0.5<x<1) during delithiating process was studied in order to optimize the electrochemical property of the Li-ion battery cathode material LiCoO₂.Li_xCoO₂ samples with different lithium contents were prepared by chemically extracting lithium from LiCoO₂ with Na₂S₂O₈. The crystal structures of these samples were then analyzed by X-ray diffraction and electron diffraction. From X-ray diffraction, the lattice parameter c of Li_xCoO₂(0.5<x<1) changes with Li content. The results of electron diffraction indicate that the ordered arrangement of lithium and vacancy in Li_xCoO₂(0.5<x<1) which cannot he detected by XRD leads to significant change in crystal structure and the corresponding crystal structure model was proposed. Phase transformation exhibits voltage plateau in the charge and discharge curves of a Li-ion battery with LiCoO₂ cathode.