The present disclosure relates to a positive electrode active material which reduces lithium by-products and improves structural stability and includes a lithium-nickel based transition metal composite oxide in which an alkaline earth metal having oxidation number of +2 is doped and a phosphate coated layer formed on the outer surface of the composite oxide. Accordingly, a second battery including the positive electrode active material has excellent capacity characteristics, and also improves structural stability during charging/discharging and prevents swelling, thereby being capable of exhibiting excellent life characteristics. Therefore, the present invention may be easily applied to industry in need thereof, and particularly to electric vehicles industry requiring high capacity and long-term life characteristics.