It is crucial to investigate the thermal degradation of lithium-ion batteries (LIBs) to understand the possible malfunction at high temperature. Herein, we investigated the effects of surface film formation on the thermal degradation of lithium cobalt oxide (LiCoO2, LCO) cathode that is one of representative cathode materials. Cycling test at 60oC exhibited poorer cycleability compared with the cycling at 25oC. Cathodes after the initial 5 cycles at 60oC (60-LCO) exhibited higher impedance compared to the cathode after initial 5 cycles at 25oC (25-LCO), resulting in the lower rate capability upon subsequent cycling at 25oC, although the capacity values were similar at the lowest C-rate of 0.1C. In order to understand degradation of the LCO cathode at the high temperature, we analyzed the cathodes surface using X-ray photoelectron spectroscopy (XPS). Among various peaks, intensity of lithium hydroxide (LiOH) increased substantially after the operation at 60oC, and the C-C signal that represents the conductive agent was distinctly lower on 60-LCO compared to 25-LCO. These results pointed to an excessive formation of cathode-electrolyte interphase including LiOH at 60oC, leading to the increase in the resistance and the resultant degradation in the electrochemical performances.