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Calculations of the magnetic domain structure (domain wall energy density, domain wall thickness, domain width and critical diameter of single domain particles) in Nd 2 Fe 14 B have been made by using its intrinsic parameters (lattice constants, magnetic moments of the atoms, Curie temperature, spontaneous magnetization and magneto-crystalline anisotropy constant). The four stages of magnetization reversal processes in oriented sintered Nd-Fe-B magnet after saturation magnetization along its easy direction are studied in detail in this paper. The results show that the nucleation of the reverse-magnetization nuclei in the surface layer of the grains of hard magnetic phase Nd 2 Fe 14 B, as well as the irreversible domain wall motion from the surface to the inside of the grains are dominant factors controlling the intrinsic coercivity of the Nd-Fe-B alloy.