The characteristics of Li-Mn-O ternary compound was studied as a cathode active material for lithium secondary batteries. Li-Mn-O cathode active materials were prepared by the reaction of γ-MnO₂(EMD) and LiOH·H₂O at 375℃ with various molar ratios. As the contents of lithium increased, the cyclic reversihility was improved. Compound with Li/Mn=3/7 molar ratio showed the best results, considering reversibility and discharge capacity at the same time. The effects of the types of MnO₂ on the cyclic behaviour were also studied. It was shown that the channel structure of MnO₂ played important roles on the cyclic behaviour of Li-Mn-O cathode. Themodynamically, the larger the channel size, the greater the discharge capacity. The α-phase, however, which has the largest [2×2] channels showed lower lithium ion diffusivity than the γ phase having [1×2]+[1×1] channels. It was thought that this behaviour was attributed to the impurities inserted in the process of synthesis of α-MnO₂, So, as the discharge rate increased, γ-phase showed better result than α phase.