This work studies the aging processes of commercial LiCoO2/mesocarbon microbeads (MCMB) cells which are cycled at 25°C, 35°C, 45°C respectively at the 0.6C charge/discharge rate and 30% depth of discharge. The capacity degradation of the cells is fast at elevated temperature, and the cycle life tested at 45°C is about a quarter of the cycling time at 25°C. The fresh and the aged cells are disassembled to characterize the morphology and the composition of electrode surface, as well as the bulk structure and the electrochemical performance of single electrode. It is found that the formation of SEI film and the polarization of the full cell lead to state of charge (SOC) shift in the cathode. The cathode SOC shift and the decay in the reversible capacity of LiCoO2 cathode dominate the aging of the full cell. The former is the prevailing aging factor at 25°C, while the latter factor becomes the leading cause of cell aging at 45°C. The unstable and thick SEI film on the cathode under elevated temperature influences the lithium ion diffusion, resulting in the increased polarization and the decreased intrinsic performance of LiCoO2 cathode. The proper range of test temperature ensuring no changes in aging mechanism and the decay rate of capacity caused by each aging factor are proposed by analyzing the performance of the full cells and the electrodes. After comparing the test results, it is concluded that the aging process at 45°C is not the same as that at room temperature.