Au -Cu nanoparticles are widely used as catalysts in different chemical reactions. Since knowing the phase diagram of nano-alloys is crucial for effective design of nano-catalysts, there have been many efforts to predict the size effect on the phase diagram of the Au -Cu system. However, reported results are inconsistent and sometimes contradictory. In this work, a CALPHAD type thermodynamic model was applied to recalculate the phase diagram of Au -Cu binary alloy nanoparticles at different sizes. The results show that decreasing particle size decreases liquidus and solidus temperatures as well as the congruent melting point. It was also found that by reduction of the particle size, the composition of the congruent alloy shifts towards the Aurich side of the phase diagram.