A micromagnetic analysis of the temperature dependence of the coercivity of sintered Nd-Fe-B permanent magnets with and without post-annealing treatment has been made based on the nucleation mechanism. Two temperature ranges in which different mechanisms control the coercivity are clearly distinguishable. From the micromagnetic point of view, the post-annealing process reduces the local effective demagnetizing field (-N_effM_s), which makes the nucleation of a reversed domain more difficult, and therefore enhances the coercivity.