A positive electrode active material includes a lithium transition metal oxide, which contains 70 mol % or more of nickel (Ni) among total metals excluding lithium and is in a form of a secondary particle formed by aggregation of primary particles. The lithium transition metal oxide has a coating layer containing boron (B). An average aspect ratio of the primary particles present on a surface of the lithium transition metal oxide is in a range of 1.8 to 6.0. The method of preparing the positive electrode active material is also provided. Additionally, a positive electrode and a lithium secondary battery which include the positive electrode active material are provided.