Stacking faults probability, structure and microstructure parameters of different phases of ball milled Fe-50%Co powders have been quantitatively evaluated by X-ray diffraction profile analysis. It is observed that after short milling time. Co is found to undergo an allotropic transformation from FCC to HCP form. The Rietveld whole profile fitting results reveal an increase of the stacking fault probability with increasing milling time and that twin faults are more prevalent than deformation ones. In addition to the reduction of crystallite size to the nanometer level and the increase of internal strain, prolonged milling leads to the formation of a BCC Fe(Co) solid solution with a high dislocations density (3.8 x 10(17) m(-2)). (C) 2010 Elsevier B.V. All rights reserved.