The nanocrystalline zinc ferrite (ZnFe2O4) powder was synthesized by high energy reactive ball milling (RM) in a planetary mill. As starting materials a mixture of commercial zinc oxide (ZnO) powder and iron oxide (Fe2O3) powder was used. The starting mixture was milled for different periods of time, up to 30 h. The milled powders were annealed for 4 h at 350 oC in order to eliminate the internal stress and to finish the solid state reaction of ferrite formation. Zinc ferrite formation was investigated by X-ray diffraction. The obtained powder has a mean crystallite size of 12 nm after 20 h of milling. Using scanning electron microscopy (SEM) the particle morphology was studied. Particles size range of the powders was also determined using a laser particle size analyser.