A high performance Li-ion thin film battery is essential for nano/microelectronics that require micro power source. Thus, the cathode of thin film battery needs to be prepared using an adequate thin film technology. In this study, the main objective was to fabricate a nanosize LiCoO2 film as the cathode of the Li-ion thin film battery. The hydrothermal process was adopted for the thin film deposition of LiCoO2. The solutions were prepared from the lithium nitrate and cobalt acetate in deionized water. LiCoO2 thin film was precipitated on a substrate during hydrothermal process and followed by annealing at desired temperatures up to 4h in air. The crystallization of LiCoO2 film was analyzed by DTA/TG and XRD. A well crystallized hexagonal LiCoO2 and cubic CO3O4 film were obtained. The surface morphology of as deposited LiCoO2 film was examined by SEM. Finally, CV test and charge/discharge tests were conducted on an assembled Li battery using the deposited LiCoO2 film by hydrothermal process.