Abstract: Ball milling induced microstructure evolution in an FeMoSiB alloy with three kinds of different phase constitutions (bcc Fe solid solution and various borides) was investigated by means of X-ray diffraction and transmission electron microscopy. Experimental evidences indicate that a same final product of bcc Fe solid solution was formed upon miling for all samples. During the transformation, the XRD peaks of Fe phase shift to lower diffraction angIes; and those of borides disappear in turn, first the metastable Fe3B and Fe23B6 and then the stable Fe2B. The observed structure evolution might originate from the transformation of ordered Fe2B phase into disordered Fe solid solution.