A positive electrode active material containing a lithium metal composite oxide composed of secondary particles formed by aggregated primary particles, comprising lithium, at least one metal element and at least one additive element, the lithium metal composite oxide having a crystal structure of layered rock salt structure and the metal element including nickel in a content of 60 to 90 atomic percent and the additive element including boron in content of more than 1.0 atomic percent and 6.0 atomic percent or less, the nickel content and the boron content each with respect to the sum of the metal element and the additive element, the porosity of the secondary particles being 8% or more and 20% or less; a non-aqueous electrolyte secondary battery containing the positive electrode active material; and a process for producing the positive electrode active material.