Effect of Co substitution for Mn on Li[Ni0.5Co x Mn1.5−x ]O4 was investigated. Co-precipitation was employed to synthesize the [Ni0.25Co x Mn0.75−x ](OH)2 as a precursor and it was fired with LiOH at 900°C for 20h in air. From Rietveld refinement of X-ray diffraction data and scanning electron microscopic examinations, it was found that the as-synthesized Li[Ni0.5Co x Mn1.5−x ]O4 were crystallized in cubic spinel structure with Fd3m space group and the final products presented spherical secondary morphology (3μm in average). Electrochemical investigation revealed that Co3+ replacement for Mn4+ gave rise to improved rate and cycling performances probably due to the improved electronic conductivity and structural stability achieved by the presence of Co3+ in the spinel Li[Ni0.5Co x Mn1.5−x ]O4.