Provided is a cathode active material containing a Ni-based lithium mixed transition metal oxide. More specifically, the cathode active material comprises the lithium mixed transition metal oxide having a composition represented by Formula I of LixMyO2 wherein M, x and y are as defined in the specification, which is prepared by a solid-state reaction of Li2CO3 with a mixed transition metal precursor under an oxygen-deficient atmosphere, and has a Li2CO3 content of less than 0.07% by weight of the cathode active material as determined by pH titration. The cathode active material in accordance with the present invention and substantially free of water-soluble bases such as lithium carbonates and lithium sulfates and therefore has excellent high-temperature and storage stabilities and a stable crystal structure. A secondary battery comprising such a cathode active material exhibits a high capacity and excellent characteristics, and can be produced by an environmentally friendly method with low production costs and high production efficiency.