Provided is a non-aqueous electrolyte-based, high-power lithium secondary battery having a long service life and superior safety at both room temperature and high temperature, even after repeated high-current charging and discharging. The battery comprises a cathode active material composed of a mixture of lithium/manganese spinel oxide and lithium/nickel/cobalt/manganese composite oxide wherein the cathode active material exhibits the life characteristics that the capacity at 300 cycles is more than 70% relative to the initial capacity, in the provision of satisfying the condition (i) regarding the particle size and the condition (ii) regarding the mixing ratio.