Nd-Fe-B/Nd(-O) interfaces were prepared by thin film technique for the investigation of relationship between interfacial state and coercivity. A Nd-Fe-B layer was deposited by UHV magnetron sputtering and the film was oxidized under low vacuum conditions. On the oxidized Nd-Fe-B layer, a Nd layer was deposited and the film was annealed at 250-650 °C. The coercivity of Nd-Fe-B layer decreased after the oxidation. However, it recovered after the deposition of Nd layer and the annealing at 350 °C. High-resolution transmission electron microscopy (HRTEM) observations revealed that an fcc NdO phase forms after the oxidation and an amorphous phase exists along the interface between Nd2 Fe14 B and Nd2 O3 phases after annealing at 350 °C. Therefore, the recovery of coercivity is related to the amorphous phase, which is considered to be formed by the transformation from NdO to Nd2 O3 and α-Nd, and the release of strain at the interface.