A positive electrode active material or non-aqueous electrolyte secondary batteries comprises a Co-containing lithium transition metal oxide containing Ni, Mn, and an arbitrary element and having a layered structure, wherein the content ratio of Ni in the lithium transition metal oxide is 75 to 95 mol %, the content ratio of Mn in the lithium transition metal oxide is equal to or greater than the content ratio of Co in the lithium transition metal oxide, the content ratio of Co in the lithium transition metal oxide is 0 to 2 mol %, the content ratio of a metal element other than Li in an Li layer in the layered structure is 1 to 2.5 mol %, and, in the lithium transition metal oxide, the half width n of a diffraction peak for (208) plane of an X-ray diffraction pattern as measured by X-ray diffraction is as follows: 0.30°≤n≤0.50°.