In recent years, significant research efforts were undertaken to enhance coercivity (Hcj) of Nd-Fe-B sintered magnets. One of the methods is Dy diffusion, which is very effective in improving Hcj while maintaining the same level of remanence (Br). Experiments reported in this work revealed that Dy diffusion differently affected Hcj of Nd-Fe-B sintered magnets at three diffusion stages: (1) from 0 to 4 h of Dy diffusion, Hcj increased rapidly from 32.3 to 36.5 kOe as Dy content increased rapidly from 0 to 0.27 wt%; (2) from 4 to 24 h, when Dy content changed from 0.27 to 0.56wt%, Hcj increased from 36.5 to 38.7 kOe; (3) from 24 to 30 h, Hcj and Dy content barely changed. Thus, extending Dy diffusion time beyond 24 h period would not yield any improvements. From 0 to 30 h, Br slightly decreased from 12.40 to 12.22 kGs. During Dy diffusion, intergranular grain boundaries became continuous and homogeneous, and some micro-defects were repaired. As a result, from 0 to 24 h, Hcj enhanced significantly while Br barely changed. Thus, the Nd-Fe-B sintered magnets with high properties (Br = 12.22 kGs, Hcj = 38.9 kOe and (BH)max = 36.87 MGOe) were obtained.