Hydrothermal lithium extraction from well-crystallized Li 2 MnO 3 crystals was studied by scanning electron microscopy (SEM) observation, chemical, and X-ray diffraction analyses. Monoclinic Li 2 MnO 3 polyhedral crystals were prepared in an LiCl flux at 650 °C. The lithium extraction was carried out in a variety of H 2 SO 4 solutions with mole ratios of H + in the solution to Li + in the solid ([H + ] l /[Li + ] s ) of 0.1, 0.15, 0.25, 0.50, 0.75, and 1.0 under a hydrothermal condition at 140 °C. The lithium extraction from Li 2 MnO 3 proceeds consuming equimolar H + in the solution by the mechanism of a Li + /H + ion exchange reaction when [H + ] l /[Li + ] s e 0.15 and by the mechanism of a mixture of the Li + /H + ion exchange reaction and lithium dissolution when 0.25 e [H + ] l /[Li + ] s e 0.75. The lithium-extracted phase preserves the crystal structure of Li 2 MnO 3 with a partial contraction of the lattice when 0.25 e [H + ] l /[Li + ] s e 0.75. SEM observation showed that the lithium extraction brings about a cleavage of the Li 2 MnO 3 particle along the (001) plane to result in a unique form in which platelike particles are stacked, preserving the polyhedral outline. High-magnification SEM observation showed that the platelike particles consist of an aggregate of subplates with nanometer thickness in parallel arrangement. Slit-shaped mesopores were formed among the subplates. γ-MnO 2 was formed as a final product when [H + ] l /[Li + ] s ) 1. A structural model is proposed to explain the mechanism of lithium extraction as well as the cleavage of the particle.