The sub-solidus phase equilibria in the NdF3-Nd2O3 system was investigated over the whole composition range by powder X-ray diffraction (XRD) and differential thermal analysis (DTA) between room temperature and 1573K. The intermediate compound neodymium oxyfluoride was found to exhibit tetragonal, rhombohedral and cubic modifications. The tetragonal phase exists over 20 to 40 m% Nd2O3 and rhombohedral phase has a fixed composition of 50 m% Nd2O3. The rhombohedral phase transforms to cubic at around 826K.