2D materials present new opportunities in fundamental research and practical applications. The unique properties of 2D materials mainly rely on the atomic structure and defects. Thus study at atomic scale is crucial for deep understanding of 2D materials and provides insights into future applications. Using aberration-corrected transmission electron microscopes, atomic resolution imaging of individual atoms has been achieved even at a low kV. Ongoing optimization of aberration correction improves the spatical resolution better than angstrom and moreover boosts the contrast of light atoms. My talk covers the research progress on atomic structure of mono- and multi-layered 2D materials, functionalized graphene and defects of 2D materials.