Fe-Cu system is a binary alloys system, nevertheless very difficult. This paper presented the milling duration influence on ball-milled Fe30Cu70 alloys. After 16 hour of milling it has been concluded that true alloying at atomic level occurs during milling. The average grain size depends by milling time. Varying the milling time changes the powder morphology, their size and structure. We found that the complete fcc Fe –Cu solid solution is formed when the grain size of Fe-bcc reach a value about 10 nm, because at this value of crystallite the free energy for interface become less than interfaces energy. The milling duration have a strongly influence on solid solubility and phases form in Fe-Cu system. The phase formation for Fe30Cu70 (mass %) has been investigated by X-ray diffraction (XRD). The mixing enthalpy (positive in this system) also depends on alloy composition.