There is intense interest in sodium-ion batteries as an alternative to lithium-ion batteries for electric storage applications because of the low-cost and abundant sodium resources. Na0.67Ni0.33xMgxMn0.67O2 compounds (x = 0, 0.02, 0.05, 0.1, or 0.15) were prepared by a solgel method and used as a cathode for sodium-ion batteries. The X-ray powder diffraction measurements demonstrated that the obtained samples have a pure P2 phase. Na0.67Ni0.23Mg0.1Mn0.67O2 delivers an initial reversible capacity of 105 mAh g(1) in the potential region from 2.0 to 4.5 V at a charge/discharge current density of 48 mA g(1). Moreover, the cyclability is improved by doping Mg. The capacity of Na0.67Ni0.23Mg0.1Mn0.67O2 can remain at approximately 84.9 mAh g(1) at a current density of 48 mA g(1) after 100 cycles. The improved high rate performance of Na0.67Ni0.23Mg0.1Mn0.67O2 was attributed to the increased lattice parameters and d spacing of the Na+ layer. Therefore, Mg-doped Na0.67Ni0.23Mg0.1Mn0.67O2 is a promising cathode for sodium-ion batteries with excellent rate and cyclic performance.