A positive active material for a rechargeable lithium battery includes a first compound represented by Chemical Formula 1, and a second compound represented by Chemical Formula 2 and having a smaller particle diameter than the first compound, wherein at least one of the first compound and the second compound includes a core and a surface layer surrounding the core: Lia1Nix1Coy1M11-x1-y1O2,  Chemical Formula 1 Lia2Nix2Coy2M21-x2-y2O2,  Chemical Formula 2 wherein M1 and M2 are each independently at least one selected from Mn, Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. The atomic concentration (at %) of nickel (Ni) with respect to the total amount of non-lithium metals is higher in the surface layer than in the core, and an amount of cation mixing is less than or equal to about 3%.