Stable spinel material Li4Ti5O12 was prepared through solid-state reaction using H2TiO3, Li2CO3 and PEG2000 as precursors. The phase purity and microstructure of the synthesized powders were studied by X-ray diffraction, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The particles are well dispersed with diameters distributed in the range of 100–150 nm. Li4Ti5O12 was doped with different mole fractions of Mg to improve the electrochemical performance of the sample at a high current rate. The best charge and discharge capacities, 145 mAh/g and 149 mAh/g, were obtained as 3 at% Mg was doped when the current density was 0.1 C. The rate property of Li4Ti5O12 could also be enhanced by the doping of Mg.