Abstract: Hydroxide coprecipitation method was used to dope Mg to the precursor to improve the cycle performance of LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials. The cathode materials were synthesized by mixing the precursor with lithium salt, ball milling and high temperature calcination. The doped and undoped cathode materials were compared by SEM, XRD and electrochemical Testing respectively. The results showed that both the doped and undoped cathode materials were standard α-NaFeO₂ layered structures with no obvious changes in particle sizes. The initial specific discharge capacity were 138.2 and 145.3 mAh/g for the cathode materials doped with 0.03 Mg and the undoped cathode materials. The specific discharge capacities were 131.1 and 119.5mAh/g separately after 50 cycles. The results indicated the cycle performance increased notably after doping Mg though the initial specific discharge capacity decreased slightly.