The coercivity of Pr2Fe14B/α-Fe magnetic nanomaterial has been drastically improved from 6.8 kOe to 26 kOe by the diffusion of the Pr68Cu32 alloy. Pr68Cu32 alloy reacts with α-Fe to form boundary phases among Pr2Fe14B grains. The boundary phases have weak magnetic properties and have a thickness comparable to the domain wall thickness of Pr2Fe14B phase. So boundary layers are very effective in pinning the motion of domain walls, leading to an increased coercivity. The coercivity of this material at 160 °C is improved from 2.7 kOe to 10 kOe by diffusion treatment.