The porous hierarchical assembly of lithium-rich Li1+xMnO 3-δ 2D nanoplates as well as isolated 0D nanocrystalline homologues has been synthesized via lithiation reactions of nanostructured manganese oxides under hydrothermal conditions. According to powder X-ray diffraction and electron microscopy, a hydrothermal LiOH treatment for nanostructured δ-MnO2 precursor produces a lithium-rich Li 1+xMnO3-δ phase with the nanoworm-like hierarchically assembled 2D nanoplate morphology. After the lithiation reaction under identical conditions, the 1D nanowires of the α-MnO2 precursor are transformed into the 0D nanoparticles of the Li 1+xMnO3-δ phase. The Mn K-edge X-ray absorption spectroscopic analysis for the lithiated materials clearly demonstrated that tetravalent manganese ions are stabilized in octahedral sites of a Li 2MnO3-type layered structure composed of edge-shared MnO6/LiO6 octahedra. From electrochemical measurements, it was found that the lithiated Li1+xMnO3-δ nanostructured materials show much superior electrode performance over the precursor manganese oxides and bulk lithium-rich manganate. The powder X-ray diffraction analyses for the electrochemically cycled derivatives clearly demonstrated that the improvement of electrode performance after lithiation can be attributed to the phase transformation to the Li-rich Li 1+xMnO3-δ phase with high structural stability. On the basis of the present experimental findings, we are able to conclude that the present phase transformation route provides a new method not only to synthesize nanostructured lithium-rich manganese oxides with controllable dimensionality and morphology but also to improve the electrode performance of nanostructured manganese oxides.