A positive active material for a rechargeable lithium battery, comprising a first compound represented by Chemical Formula 1 in form of a secondary particle in which a plurality of primary particles is assembled and a second compound represented by Chemical Formula 2 in form of a secondary particle in which a plurality of primary particles is assembled, wherein the secondary particle of the second compound has a smaller average particle diameter than the secondary particle of the first compound measured by using a scanning electron microscope, at least one of the secondary particle of the first compound and the secondary particle of the second compound comprises a core and a surface layer surrounding the core, wherein a Ni atom concentration (at%) of the metals except Li of the surface layer is higher than a Ni atom concentration (at%) of the metals except Li of the core, and a cation mixing is less than or equal to 3%, wherein cation mixing means a ratio (%) of a portion of nickel ions (Ni 2+) mixed to a Li site within the crystal lattice of the positive active material measured by a X-ray diffraction analysis and determined by Rietveld analysis: [Chemical Formula 1] Li a1 Ni x1 Co y1 M 1 1-x1-y1 O 2 wherein, in Chemical Formula 1, 0.9 ≤ a1 ≤ 1.05, 0.3 ≤ x1 ≤ 0.95, 0.1 ≤ y1 ≤0.3, and M 1 is at least one metal element selected from Mn, Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce, [Chemical Formula 2] Li a2 Ni x2 CO y2 M 2 1-x2-y2 O 2 wherein, in Chemical Formula 2, 0.9 ≤ a2 ≤ 1.05, 0.3 ≤ x2 ≤ 0.95, 0.1≤ y2 ≤ 0.3, and M 2 is at least one metal element selected from Mn, Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. a first compound represented by Chemical Formula 1 in form of a secondary particle in which a plurality of primary particles is assembled and a second compound represented by Chemical Formula 2 in form of a secondary particle in which a plurality of primary particles is assembled, wherein the secondary particle of the second compound has a smaller average particle diameter than the secondary particle of the first compound measured by using a scanning electron microscope, at least one of the secondary particle of the first compound and the secondary particle of the second compound comprises a core and a surface layer surrounding the core, wherein a Ni atom concentration (at%) of the metals except Li of the surface layer is higher than a Ni atom concentration (at%) of the metals except Li of the core, and a cation mixing is less than or equal to 3%, wherein cation mixing means a ratio (%) of a portion of nickel ions (Ni 2+) mixed to a Li site within the crystal lattice of the positive active material measured by a X-ray diffraction analysis and determined by Rietveld analysis: [Chemical Formula 1] Li a1 Ni x1 Co y1 M 1 1-x1-y1 O 2 wherein, in Chemical Formula 1, 0.9 ≤ a1 ≤ 1.05, 0.3 ≤ x1 ≤ 0.95, 0.1 ≤ y1 ≤0.3, and M 1 is at least one metal element selected from Mn, Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce, [Chemical Formula 2] Li a2 Ni x2 CO y2 M 2 1-x2-y2 O 2 wherein, in Chemical Formula 2, 0.9 ≤ a2 ≤ 1.05, 0.3 ≤ x2 ≤ 0.95, 0.1≤ y2 ≤ 0.3, and M 2 is at least one metal element selected from Mn, Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce.