Using first-principles calculations, we successfully investigate the electrochemical performance of the monoclinic NaMnO2 for the sodium ion batteries. NaMnO2 possesses a voltage window of 3.54-2.52 V and theoretical reversible capacity of 136 mAh g(-1). Besides, we find that the metallicity of the monoclinic NaMnO2 gradually increases during Na extraction. Moreover, the computational Na migration energy barrier in the monoclinic NaMnO2 is 0.18 eV, ensuring ideal conductivity and reversible capacity. Although the Jahn-Teller distortion effects limit the enhancement of the reversible capacity of the monoclinic NaMnO2, it is still a right cathode material for the sodium ion batteries. The computational results are well in consistent with the experimental investigations.