Convergent-beam electron diffraction (CBED), which is the most powerful technique to examine nanometer-scale local crystal symmetries [1,2], has been applied to functional materials with various types of phase transitions, such as a ferroelectric-like transition in metal [3], a ferroaxial transition [4], and ferroelectric transitions [5][6][7][8][9][10]. Nanoscale local structures of BaTiO3 and KNbO3, which exhibit the same series of structural phase transitions from the paraelectric cubic phase to three ferroelectric phases (tetragonal, orthorhombic, and rhombohedral), were investigated by the combined use of scanning transmission electron microscopy (STEM) and CBED (called STEM-CBED or 4D-STEM) [7][8][9][10]. It was revealed that the structures of the cubic, tetragonal, and orthorhombic phases are formed as averages of the polar nanostructures of the lowest temperature phase. This indicates the order-disorder-type behavior of their structural phase transitions [7][8][9][10]. Recently, the electric field response of the rhombohedral polar nanostructures was examined by the 4D-STEM technique [11]. By applying an electric field, significant changes in the distributions of the polar nanostructures. 4D-STEM is advantageous for the analysis of such nanoscale local structural variations related to novel physical properties.CBED was also applied to quantitative refinements of local crystal structures and electrostatic potentials fully based on the dynamical diffraction theory [12][13][14][15][16][17][18]. Toward the quantitative analysis of 3D nanostructures using CBED or 4D-STEM, functions to deal with nanostructures in the depth and lateral directions of the electron probe have been implemented into our CBED pattern simulation code MBFIT [12,13], which is based on the Bloch wave formulation. The scattering matrix formulation [19] was used to calculate CBED intensities from the stacking of different structures in the beam direction, together with parallel computation using OpenMPI. The phase of the incident electron wave at the boundary was set to reproduce the size and position of the coherent electron nanoprobe [17]. Quantitative refinement of nanostructures and interfaces using 4D-STEM data is discussed in the talk.The authors would like to thank Emer. Prof. M. Tanaka and Prof. M. Terauchi for their continuous support. The present study was partially supported by JSPS KAKENHI Grant numbers 18H03674, 18K18731 and 22H01150.