Li2MnO3 is the parent compound of the well-studied Li-rich Mn-based cathode materials xLi2MnO3·(1-x)LiMO2 for high-energy-density Li-ion batteries. Li2MnO3 has a very high theoretical capacity of 458 mA h g−1 for extracting 2 Li. However, the delithiation and lithiation behaviors and the corresponding structure evolution mechanism in both Li2MnO3 and Li-rich Mn-based cathode materials are still not very clear. In this research, the atomic structures of Li2MnO3 before and after partial delithiation and re-lithiation are observed with spherical aberration-corrected scanning transmission electron microscopy (STEM). All atoms in Li2MnO3 can be visualized directly in annular bright-field images. It is confirmed accordingly that the lithium can be extracted from the LiMn2 planes and some manganese atoms can migrate into the Li layer after electrochemical delithiation. In addition, the manganese atoms can move reversibly in the (001) plane when ca. 18.6% lithium is extracted and 12.4% lithium is re-inserted. LiMnO2 domains are also observed in some areas in Li1.63MnO3 at the first cycle. As for the position and occupancy of oxygen, no significant difference is found between Li1.63MnO3 and Li2MnO3.