Phase transformation of layered orthorhombic Li1-xMnO2 (o-Li1-xMnO2, 0≤x≤0.95) during the first charging process (lithium ion deintercalation in electrochemical system) was investigated using ex-situ high-resolution X-ray diffraction and Rietveld refinement. The results reveal that the increasing speed of the o-LiMnO2 cathode’s voltages is related to the change of its microstructure. And a succession of phase transformation of the o-LiMnO2 occurs in the first lithium deintercalation process. Rock-salt typed lithium manganese oxides emerge followed by the appearance of the spinel-like typed, which confirms the phase transition process from theoretical computation and crystallography conjecture in literatures. Only the above-stated type lithium manganese oxides (originated from o-LiMnO2) exist in the final product. Lithium ions extract from lattices of the rock-salt typed and the spinel-like typed lithium manganese oxides; both the lattice parameters become smaller and their microstructures change with the charge proceeding