The present invention relates to a positive electrode active material, and to a positive electrode and to a lithium secondary battery comprising same, and more particularly, to a positive electrode active material in which an active material having superior thermal stability is employed and two types of active materials having different particle sizes are used to increase the volume density of the positive electrode active material. The present invention also relates to a positive electrode and to a lithium secondary battery comprising the positive electrode active material. The positive electrode active material of the present invention comprises a small diameter active material having a mean diameter of 0.5 [mu]m and a maximum diameter of 1 [mu]m or smaller, and a large diameter active material having a mean diameter of 5 to 20 [mu]m and a maximum diameter of 100 [mu]m or smaller.; The composite positive electrode active material of the present invention is provided with improved packing density as the large diameter active material and the small diameter active material are mixed therein at a predetermined grain size ratio and weight ratio. The composite positive electrode active material of the present invention contains high-stability/high-conductivity materials to achieve improved volume density, discharge capacity, thermal stability and high-rate discharge properties.