A method of preparing a positive electrode active material which includes forming a pre-sintered mixture by adding a reaction mixture including a lithium raw material and a nickel-manganese-cobalt precursor to a first crucible and performing a primary heat treatment at a temperature of 500° C. to 800° C., and, after discharging the pre-sintered mixture from the first crucible, adding the pre-sintered mixture to a second crucible and performing a secondary heat treatment at a temperature of 700° C. to 1,000° C. to form a lithium nickel manganese cobalt-based positive electrode active material, wherein a volume of the pre-sintered mixture formed after the primary heat treatment is 20% to 50% of a volume of the reaction mixture added to the first crucible.