The microstructure simulation of spinodal decomposition was carried out in the isothermally-aged Cu-Ni-Fe alloys using the phase field method. The numerical simulation was based on a solution of the Cahn-Hilliard partial differential equation by the finite difference method. The calculated results were compared to those determined by atom-probe field ion microscope analyses of the solution treated and aged alloys. Both the numerically simulated and experimental results showed a good agreement for the concentration profiles and microstructure evolution in the aged Cu-Ni-Fe alloys. The aging process caused the change of initial morphology, iregular and interconected, to an equiaxial shape of the decomposed Ni-rich phase aligned in the elasticallysoftest crystallographic direction <100> of Cu-rich matrix.