The changes in nickel ferrite (NiFe2O4) due to high-energy milling in a stainless steel vial have been investigated by Mossbauer spectroscopy. X-ray diffraction (XRD). and thermal analysis. The milling process reduces the average crystallite size of NiFe2O4 to the nanometer range. The Mossbauer spectra show the formation of metallic iron and of a phase containing Fe2+ ions. The fraction of the reduced products increases with increasing milling time. The range of thermal stability of the metastable milled reduction products has been determined by studying their response to changes in temperature.