The effect of Dy addition on the microstructure and magnetic properties of the sintered NdFeB magnets was investigated. The results of the microstructure analysis showed that Dy-free and Dy-doped samples are composed of $Nd_2Fe_{14}B$ (P42/mnm) and a trace of Nd-rich phase. Dy addition reduces significantly the pole density factor of (004), (006) and (008) crystal faces as estimated by the Horta formula. Accordingly, the coercivity of the Dy-doped sample increases from 2038 $kA{cdot}m^{-1}$ up to 2288 $kA{cdot}m^{-1}$. The $H_{cj}(T)/M_s(T)$ versus $H^{min}_N/M_s(T)$ (Kronm$ddot{u}$ller-plot) behavior shows that the nucleation is the dominating mechanism for the magnetization reversal in these two kinds of magnets, and two microstructural parameters of ${alpha}_k$ and $N_{eff}$ are obtained. The Kronm$ddot{u}$ller-Plot gives evidence for an increase of the ${alpha}_k$ responsible for an increase of the coercivity as the result of the increase of the magnetic field as the magnetic domain reversed.