Improving thermal stability is one of the most serious issues concerning Ni-based cathode materials for Li ion batteries. To increase the capacity for maintaining good thermal stability, we substitute Mo for Mn. We apply differential scanning calorimetry (DSC), thermal desorption spectrometry-mass spectrometry (TDS-MS), and X-ray diffraction (XRD) to elucidate the effect of Mo substitution. The application of DSC indicates that Mo substitution effectively reduces the exothermic reaction between the cathode and electrolyte. The applications of TDS-MS and XRD indicate that Mo substitution suppresses crystal structure change and reduces the oxygen release that accompanies heating below 350 °C. However, the discharge capacities of Mo-substituted samples are lower than that of sample without Mo substitution. We optimize the Li/transition metal ratio and obtain Li1.10Ni0.8Mn0.06Co0.1Mo0.04O2+δ , which satisfy both high capacity and high thermal stability requirements.