Structure design and performance regulation of functional materials are the cutting-edge hot topic in the field of materials science and condensed mater physics. Constructing hierarchical structures with functional units recently has become a new paradigm to improve the functionality of functional materials and explore new physical phenomena. Understanding the origin of physical properties of functional materials constructed by functional units requires us to precisely characterize the structure, configuration and spatial patterns of functional units, and their couplings. Aberration-corrected transmission electron microscopy has proven to be powerful in revealing the atomic structure, chemistry and electronic configuration of the functional materials with low symmetry and complex compositions, which provides a new avenue to reveal the functional units and their spatial patterns with high precision from different aspects and finally establish the structure-propertys relationship. In this paper, we summarize the inherent characteristics of typical functional units with different sizes, and the hierarchical structures constructed by functional units at the picoscale, by which the relationship between structures and functionality is revealed. The breakthrough and development of aberration-corrected transmission electron microscopy lays a solid foundation for understanding the origin of functionality of new materials constructed by functional units.