Spinel ferrite ZnFe2O4 powders were synthesized directly from ZnO and α-Fe2O3 mixtures by high-energy mechanical milling. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were used to investigate the chemical reaction processing at different milling time. It has been found that the solid reactions between metal oxides are characterized by stages. Once the solid chemical-synthesized reaction is initialized, it proceeds quickly and can be completed in very short time. Grain sizes of ZnFe2O4 are less than 10 nm.