Phase behaviors of binary alloys with an embedded atom model potential are investigated using the thermodynamic perturbation theory. The free energies of the liquid and solid phases are computed using the fundamental measure density functional theory and accurate approximations to the hard-sphere mixture correlation functions. The method is applied to calculate the Au-Cu alloy phase diagram. To improve the accuracy of the computed phase diagram, we developed a systematic approach to optimize the model potential of Au-Cu by adjusting the melting temperature of the pure Au to its experimental one. With such an optimized potential the computed Au-Cu alloy phase diagram is in good agreement with the experimental one for the whole composition range.