Structure, electronic properties and Na ion migration dynamics of layered-Na2MnO3 are studied from first principles calculation. Results show that the lattice constant of the Na2MnO3 is much larger comparing with that of the Li2MnO3. This gives rise to two advantages of the Na2MnO3 compound as cathode materials for Na ion batteries. Firstly, the energy gap of the Na2MnO3 (1.18 eV) is smaller than that of the Li2MnO3 (1.62 eV), which is beneficial to the rate performance. Secondly, the large Na-O interlayer distance enables the Na ion diffusion with comparable energy barriers to that of Li ion, although Na ion has much larger atomic radius. Very interestingly, it is also found that Na ion in the transition metal (TM) layer is energetically more favorable in the Na2MnO3 compound, comparing with that of Li in the TM layer in Li2MnO3, which is beneficial to the structural stability during the charge/discharge process. Our results provide reasonable explanations to the experimentally observed good performance of the Na2MnO3 as cathode material for Na ion batteries.