A cathode active material including a composite transition metal oxide including: sodium; a first transition metal; and a second transition metal, wherein the composite transition metal oxide has a first diffraction peak corresponding to a Miller index of (003) and derived from a layered rock salt structure, and a second diffraction peak corresponding to a Miller index of (104) and derived from a cubic rock salt structure in an X-ray powder diffraction (XRD) pattern, wherein an intensity ratio (I1/I2) of the first diffraction peak to the second diffraction peak is about 7 or greater.