Provided are a mixed cathode active material including layered structure lithium manganese oxide expressed as Chemical Formula 1 and a second cathode active material having a plateau voltage profile in a range of 2.5 V to 3.3 V, and a lithium secondary battery including the mixed cathode active material. The mixed cathode active material and the lithium secondary battery including the same may have improved safety and simultaneously, may be used in an operating device requiring the foregoing battery by widening a state of charge (SOC) range able to maintain power more than a required value by allowing the second cathode active material to complement low power in a low SOC range.