Positive electrode active material for rechargeable lithium battery and rechargeable lithium battery

A positive electrode active material for a rechargeable lithium battery, and a rechargeable lithium battery including the same are provided. The positive electrode active material includes a lithium nickel-based composite oxide wherein the positive electrode active material is in a form of secondary particles in which a plurality of primary particles are aggregated and at least a portion of the primary particles are radially arranged, in a cross-section of the secondary particles, a number ratio of the primary particles having a cross-sectional area of less than about 0.1 μmis greater than or equal to about 65%, and a full width at half maximum (FWHM) of the peak corresponding to the (003) plane in the X-ray diffraction analysis for the positive electrode active material is less than or equal to about 0.125.