The nanocrystalline Cu-5wt.%Cr alloy powders were prepared by mechanical alloying. The structural changes were characterized by X-ray diffraction (XRD) technique. A thermodynamic analysis was carried out to predict the change in the solubility limit of this system. It was found that the energy resulting from the MA process is sufficient to increase the solid solubility of immiscible Cr-Cu system. The solid solubility may be extended up to 5 wt.% Cr in Cu after 20 h milling. The formation of the supersaturated solid solution leads to the decrease of Cu lattice parameter. However, it decomposes with the further increase of the milling time, which leads to the increase of Cu lattice parameter.