Different solid-state reactions, i.e. Fe + SiC --> Fe3C + Fe(Si), and Fe + SiC --> Fe3Si + Fe2Si + C, were found in mechanically alloyed nanocrystalline Fe/SiC composites induced by prolonged milling or heat treatment, respectively. The solid-state reaction between nanocrystalline iron and SIC upon heating is greatly enhanced when compared with that between bulk iron and SiC. It is believed that the prolonged milling-induced reaction is related to the changed thermodynamics and kinetics while the heat-treatment-induced reaction, completed during a short time, is attributable to the changed reaction kinetics.