This study provides the influence of microstructural change of the interface between Nd2Fe14B and Nd-O phases on coercivity of Nd-Fe-B thin films during annealing at low temperature (∼350 °C). All films were prepared by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under Ar gas atmosphere (O2 content; ∼2 Vol.ppm). Then, the films were annealed at 250-350 °C under UHV condition. After oxidation, the coercivity of Nd-Fe-B film decreased to around 40% of the coercivity of as-deposited Nd-Fe-B film. The Nd-rich phase changed from α-Nd to amorphous Nd(-O), and the interface of Nd2Fe 14B/Nd(-O) became rough. In the Nd-Fe-B films oxidized and subsequent annealed at 350 °C, the coercivity decreased to around 20%. In the films, poly crystalline hcp Nd2O3 phase crystallized in Nd-rich phase, and there were some steps at the surface of Nd2Fe 14B phase contacting with hcp Nd2O3 phase. Regardless of crystal orientation of Nd2Fe14B, the microstructural changes of the interface described above were observed.