Positive electrode active material for lithium secondary battery and method of preparing the positive electrode active material

A positive electrode active material includes a lithium transition metal oxide, which is in the form of a secondary particle formed by aggregation of primary particles and is represented by Formula 1, wherein the lithium transition metal oxide has a crystalline size of 160 nm or less and an average particle diameter of the primary particle of 0.6 μm or more. A preparation method thereof is also provided.