A birnessite-type sodium manganese oxide with a layered structure was synthesized by the reaction of Mn(NO₃)₂ solution with a mixed solution of H₂O₂ and NaOH at room temperature. This sodium manganese oxide was hydrothermally treated in neutral, acidic and alkaline solutions at 150°C. The original samples which were synthesized at room temperature and hydrothermally treated samples were characterized by XRD, DTA-TG, FT-IR spectroscopy and chemical analysis. The formation of the birnessite-type sodium manganese oxide is dependent on the mixed mole ratio of NaOH/Mn(NO₃)₂. The fraction of the birnessite phase increased with increase in mixed mole ratio of NaOH/Mn(NO₃)₂. The birnessite-type manganese oxide has a basal spacing of 1.004nm before air-drying, but it changes to 0.724nm after the air-drying at room temperature. A well crystallized birnessite phase with a basal spacing of 0.724nm was obtained by hydrothermally treating the birnessite-type sodium manganese oxide prepared at room temperature with a NaOH solution.