Preparation of nanosized NiFe2O4 particles by mechanochemical reaction(NiO þ a-Fe2O3) and subsequent thermal treatment was investigated using X-ray diffraction (XRD). Thermal treatment of the as-milled powder at 700℃ for 1h led to the formation of NiFe2O4 nanoparticles with an average crystal size of about 23nm. Effect of thermal treatment temperature on the crystal size of the nanoparticles was studied. The mechanism of nanoparticles growth was primarily discussed. The activation energy of NiFe2O4 nanoparticle formation during calcination was calculated to be 16.6 kJ/mol.