A 3V cathode material for lithium ion batteries, Li0.33MnO2, was synthesized by solid-state reaction. Two Mn crystallographic positions, Mn(1) and Mn(2), were determined by X-ray diffraction analysis. The [Mn(2)O6] octahedron had a lower symmetrical degree than that of [Mn(1)O6], which was attributed to the geometrical effects of the non-symmetrical environment around Mn(2). Li0.33MnO2 delivered a reversible discharge capacity ～140mAhg-1. In situ synchrotron diffraction clearly showed a reversible phase transition of Li0.33MnO2 during electrochemical process. The analysis of X-ray absorption near edge spectroscopy observed the conversion of Mn4+ to Mn3+ with Li+ intercalation into Li0.33MnO2, accompanied by the formation of more severely distorted [MnO6] octahedron.