AbstractX‐ray diffraction analysis, scanning electron microscopy and thermodynamic calculations in the Miedema's model were used to study the extension of solid solubility in copper‐lead immiscible system prepared by mechanical alloying. The result shows that the nanocrystalline supersaturated solid solution formed during the alloying process. The Gibbs free energy change in this system during the formation of solid solution is calculated and shows to be positive, which means that a thermodynamic barrier exists for the formation of this alloy system in solid state. It has been found that the additional energy stored in nanocrystalline copper‐lead alloy during the alloying process as a result of crystallite size reduction and increased dislocation density, would be high enough to overcome the thermodynamic barrier in the formation of a solid solution.