Lithiated transition metal oxides are known for their use as cathodes in lithium-ion batteries. Most studies since 1990 have focused on LiCoO2, LiNiO2, and LiMn2O4. Over the last few years, there have been considerable experimental and theoretical studies conducted on LiMnO2, whose layered form bears a structural similarity to the parent compound, NaMnO2. First principle ab initio studies have therefore been conducted in this system. Results of the simulation show that the monoclinic phase of the compound is stable in the antiferromagnetic state similar to the experimental studies. Density of states and band structure calculations reveal that the antiferromagnetic monoclinic structure is a semiconductor with a bandgap of 1.3 eV. Both the ferromagnetic and antiferromagnetic states of the metastable hexagonal polymorph are metallic. Arguments based on the atomic size and magnetic interactions have been made to justify the stability of the monoclinic phase of NaMnO2. It is envisaged that these fundamental studies on the structure and electronic properties of NaMnO2 will play an important role in understanding the stability of the layered form of NaMnO2. (C) 2003 The Electrochemical Society.