The grain boundary diffusion process (GBDP) using a heavy rare earth elements (HRE) such as Dy and Tb is known as an effective method to enhance the coercivity of Nd–Fe–B sintered magnets without reducing remanence. This process has been industrially implemented to manufacture Nd–Fe–B based sintered magnets with high coercivity and high remanence. In this process, Dy is considered to diffuse through grain boundaries (GBs) to form (Nd1−xDyx)2Fe14B shells surrounding the Nd2Fe14B grains and the higher anisotropy field of the Dy-rich shell is considered to suppress the nucleation of reverse domains at low magnetic field. Although there are several investigations on the microstructure of HRE GBDP Nd–Fe–B magnets, no paper addressed the origin of the asymmetric formation of HRE rich shells. Based on detailed analysis of facet planes of core/shell interfaces, we propose a mechanism of the faceted core/shell microstructure formation in the GBDP sintered magnets. We believe that this gives new insights on understanding the coercivity enhancement by the GBDP. Display omitted •Faceting was observed at the interfaces of cores and shells.•The core/shell interfaces are sharp with an abrupt change in Dy concentration.•Meting occurs at the interfaces of metalic Nd-rich/Nd2Fe14B phases above 685°C due to eutectic reaction.•Solidification of Dy-enriched liquid phase from 900°C can result in the shell formation. Dysprosium enriched shell structure formed by the grain boundary diffusion process (GBDP) of a sintered Nd–Fe–B magnet was characterized by using scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Faceted core–shell interfaces with an abrupt change in Dy concentration suggest the Dy-rich shells are formed by the solidification of the liquid phase during cooling from the GBDP temperature. The Nd-rich phases are almost free from Dy, and their quantity near the surface of a bulk sample is much higher than that in the center, indicating that a higher fraction of a liquid phase exists near the surface during processing at 900°C. These microstructural features are explained on the basis of the phase equilibrium between Nd and Nd2Fe14B at the processing temperature and subsequent cooling. Based on the results, we discuss the coercivity enhancement by the GBDP using Dy vapor.