Mechanical alloying (MA) of Fe-B alloy systems using a conventional ball mill has been performed. The structure change of Fe100-xBx for the composition range 10 less-than-or-equal-to x less-than-or-equal-to 90 has been investigated using X-ray diffractometry, thermal analysis, transmission electron microscopy, scanning electron microscopy and Mossbauer spectroscopy. According to the X-ray diffraction analysis, the sequences of transformation have been classified into three groups, x less-than-or-equal-to 20, 25 less-than-or-equal-to x less-than-or-equal-to 35 and x greater-than-or-equal-to 50 for Fe100-xBx. The amorphous phase whose composition is nearly x = 30, first appeared for the whole composition range. This amorphous phase changed into a tetragonal Fe2B-like compound on further milling for 1000 h. On further milling of the x = 50 sample, the Fe2B compound phase disappeared and orthorhombic FeB compound appeared.