A positive electrode active material capable of improving an output performance of a nonaqueous electrolyte secondary battery is provided. A positive electrode active material of a nonaqueous electrolyte secondary battery 1 contains a first positive electrode active material and a second positive electrode active material. In the first positive electrode active material, the content of cobalt is 15% or more on an atomic percent basis in transition metals. In the second positive electrode active material, the content of cobalt is 5% or less on an atomic percent basis in transition metals. An average secondary particle diameter r1 of the first positive electrode active material is smaller than an average secondary particle diameter r2 of the second positive electrode active material.