Li[Ni(1/3−z)Co(1/3−z)Mn(1/3−z)Mg z ]O2 (z =0, 0.04) positive electrode materials were synthesized via a co-precipitation method. These materials have α-NaFeO2 ( R 3 ¯ m ) structure, as confirmed by X-ray diffraction (XRD) studies. Cation mixing in Li layer seemed to be decreased by Mg substitution as examined by Rietveld refinements of XRD data. Spherical morphologies were observed for the as-synthesized final products by scanning electron microscopy. Their electrochemical properties during charge and discharge were discussed. When magnesium ions are substituted, the initial reversible capacity reduced. However, the substitution for Mn sites in Li[Ni1/3Co1/3Mn1/3]O2 did not decrease the capacity because Mn sites substitution did not result in loss of electroactive elements in the compound. Differential scanning calorimetric studies showed the exothermic peaks of the charged electrode Li[Ni(1/3−z)Co(1/3−z)Mn(1/3−z)Mg z ]O2 (z =0.04) were significantly smaller than that of Li[Ni1/3Co1/3Mn1/3]O2, which means that thermal stability was greatly improved by Mg substitution even at highly delithiated state.