The local structures of the immiscible Fe100-xCux alloys (x=0,10,20,40,60,80 and 100)produced by mechanical alloying have been investigated by XAFS.For the Fe100-xCux(x≥40) alloys,the local structures around Fe atoms change from bcc structure to fcc one,and the Cu atoms maintain the original coordination geometry after milling for 160 h. On the contrary, the local structures around Cu atoms in both Fe80 Cu20and Fe90 Cu10 alloys appears a transition from fcc to bcc structure.We found that the disorder factor σ(0.0099nm)of fcc Fe—Cu phase is larger than that (0.0081nm) of bcc Fe—Cu phase,and the σ(0.0099nm) around Fe atoms is larger than that (0.0089nm) of Cu in the Fe100-xCux(x≥40) alloys. This suggests that the mechanically alloyed Fe100-xCux alloys is not a homogeneous supersaturated solid solution,but consists of Fe-rich and Cu-rich regions for various compositions.A possible mechanism for bcc-to-fcc and fcc-to-bcc changes in Fe100-xCux alloys is discussed in relation to the interdiffusion and transition induced by the ball milling.