Monoclinic layered Li2MnO3 has been extensively investigated due to its large discharge capacity. However, the poor cycling stability hinders its application as a cathode material of lithium-ion batteries. Herein we present a new strategy of sodium substitution for partial lithium to significantly enhance the cycling stability of Li2MnO3 through a conventional solid state reaction. In the electrochemical window of 2.0–4.6 V vs. Li/Li+, Li1.90Na0.10MnO3 delivers an initial discharge capacity of 181 mAh g−1 with an excellent capacity retention of 99.3% after 45 cycles at 1/10 C, and 161 mAh g−1 with a capacity retention of 98.6% after 100 cycles at 1/2 C. Sodium substitution for partial lithium is promising to make Li2MnO3 be practically applied at a low current density.