In order to explore the positive material with excellent cycling ability and good safety, the effects of co-mixed ratios(10∶0、8∶2、7∶3、6∶4、5∶5、0∶10) of LiNi_0.4Co_0.2Mn_0.4O₂ with LiMn₂O₄ as the positive material of lithium ion battery on the first discharging properties, cycling abilities, rate capabilities, impedance spectroscopy and cyclic voltammetry were investigated. The co-mixed electrode materials were also characterized by SEM. The results indicated that the co-mixed ratio affected the electrochemical performances of the co-mixed materials. The materials with the ratios of 8∶2, 7∶3 and 6∶4 showed higher specific volumetric capacity, better cyclic and rate performances than that of LiNi_0.4Co_0.2Mn_0.4O₂ and LiMn₂O₄ materials. The co-mixed material with the ratio of 8∶2 is the best.