A positive electrode active material for a non-aqueous electrolyte secondary battery, which enables a non-aqueous electrolyte secondary battery to achieve great battery characteristics, and a process for producing the material, as well as a non-aqueous electrolyte secondary batter. Provided is a positive electrode active material for a non-aqueous electrolyte secondary battery, where secondary particles of lithium-nickel-cobalt-manganese composite oxide particles are composed of outer-shell parts with a high manganese concentration and central parts with a low manganese concentration, which are lower in manganese concentration than the outer-shell parts, primary particles constituting the outer-shell parts of the secondary particles with the high manganese concentration are composed of outer peripheral parts with a low manganese concentration and inner parts with a high manganese concentration, and the ratio of the manganese concentration in the inner part to the manganese concentration in the outer peripheral part is 1.5 to 2.5 in the primary particles.