Lithium-excess manganese layered oxides, which are commonly described in chemical formula 0.5Li₂MnO₃·0.5LiMn_0.5Ni_0.5O₂, were prepared by low-heating solid state reaction. The reaction mechanisms of synthesizing precursors, the decomposition mechanism, and intermediate materials in calcination were investigated by means of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The major diffraction patterns of 0.5Li₂MnO₃·0.5LiMn_0.5Ni_0.5O₂ powder calcinated at 720℃ for 15 h are indexed to the hexagonal structure with a space group of R3m, and the clear splits of doublets at (006)/(102) and (108)/(110) indicate that the sample adopts a well-layered structure. FESEM images show that the size of the agglomerated particles of the sample ranges from 100 to 300 nm.