The nanostructures of the ball milled FeCo particles were characterized as functions of the ball milling time ( t ) using quantitative X- ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) analysis techniques.The results show that the nanocrystalline bcc FeCo particles are available using carbonyl iron and cobalt powders as the start materials during the high-energy ball milling.At the early stage of ball milling, Co powders are easily mashed into nanocrystalllites, by which the surface of the larger Fe particles of about 80- 150 nm is coated.With t increasing, the refinement of grain size and the incorporation of defects in cluding dislocations, disclinations and grain boundaries happen, and then FeCo alloy with a certain layered structure is formed, finally the layered structure disappears with the formation of isotropic grains having a steady state grain size in the nanometer regime after a certain period of t.