The microstructure simulation of spinodal decomposition was carried out in the isothermally-aged Cu-Ni and Cu-Ni-Fe and Cu-Ni-Cr 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 in the aged Cu-Ni, Cu-Ni-Fe and Cu-Ni-Cr alloys. A very slow growth kinetics of phase decomposition was observed to occur in the aged Cu-Ni alloys. The morphology of decomposed phases consists of an irregular shape with no preferential alignment in any crystallographic direction at the early stages of aging in all the aged alloys. In the case of the aged Cu-Ni-Fe alloy, a further aging caused the change of initial morphology to an equiaxial shape of the decomposed Ni-rich phase aligned in the elastically-softest crystallographic direction <100> of Cu-rich matrix. The growth kinetics rates of phase decomposition in Cu-Ni-Fe and Cu-Ni-Cr alloys are appreciably faster than that in Cu-Ni alloys.