LiCoO2 is commercially available cathode electrode materials of lithium-ion batteries. In an attempt to improve the performance of lithium batteries with enhanced safety, LiCo1−x Mg x O2 was synthesized by the solid-state reaction method. The crystal structure was analyzed by X-ray diffraction and Rietveld refinement. LiCo1−x Mg x O2 give a single phase of a layered structure with the space group R-3m of hexagonal systems for x ≤0.11. A second phase of MgO was observed in LiCo1−x Mg x O2 for x ≥0.13. The calculated cation–anion distances and angles from the Rietveld refinement were changed with Mg content in samples. With the increase in Mg content in LiCo1−x Mg x O2, distances between CoO2 slabs were increased. The electrical conductivities of sintered samples were measured at room temperature by the Van der Pauw method. The electrical conductivities of LiCo1−x Mg x O2 increased with Mg content. On the basis of the Hall effect analysis, the increase in electrical conductivities with Mg content is believed due to the increased carrier concentrations, while the carrier mobility was almost invariant with the Mg content. The electrochemical performance of LiCo1−x Mg x O2 was evaluated by coin cell test.