Temperature programmed reduction (TPR) method was introduced to analyze the structural change and thermal stability of Li x CoO 2 (LCO) cathode material. The reduction peaks of delithiated LCO clearly represented the different phases of LCO. The reduction peak at a temperature below 250 C can be attributed to the transformation of CoO 2–like to Co 3 O 4–like phase which is similar reduction patterns of CoO 2 phase resulting from delithiation of LCO structure. The 2 nd reduction peak at 300~ 375 C corresponds to the reduction of Co 3 O 4–like phase to CoO–like phase. TPR results indicate the thermal instability of delithiated LCO driven by CoO 2–like phase on the surface of the delithiated LCO. In the TPR kinetics, the activation energies (Ea) obtained for as-synthesized LCO were 105.6 and 82.7 kJ mol-1 for T m_H1 and T m_H2, respectively, whereas E a for the delithiated LCO were 93.2, 124.1 and 216.3 kJ mol-1 for T m_L1, T m_L2 and T m_L3, respectively. As a result, the TPR method enables to identify the structural changes and thermal stability of each phase and effectively characterize the distinctive thermal behavior between as-synthesized and delithiated LCO.