In this study, oxygen vacancies are successfully introduced into Li2MnO3 by low-temperature reduction. Electron paramagnetic resonance is used to detect oxygen vacancy and the results state clearly that oxygen vacancies indeed exist in the reduced samples, and R-LMO-40 shows the highest content. The relationships between oxygen vacancies content and the electrochemical performance of Li2MnO3 as cathodes for lithium ion batteries are also investigated. The oxygen-deficient samples show much better electrochemical performance compared to the pristine Li2MnO3, especially R-LMO-40 with an optimized content of oxygen vacancies. At the rates of 0.5 C, 1 C, 2 C and 5 C, R-LMO-40 delivers discharge capacities of 143.1, 134.0, 126.1 and 90.9 mAh g(-1), respectively. After cycling for 50 cycles, there is nearly no capacity decay. These better electrochemical performances are believed to be related to the characteristics of the oxygen vacancies. Electrochemical impedance spectroscopy analysis clearly indicates that oxygen vacancies can suppress the charge transfer resistance and increase lithium ions diffusion capability. These findings reveal that oxygen vacancies can be utilized to enhance the electrochemical performances of Li-rich oxide cathode materials.