It has been reported that this alloy exhibits excellent mechanical and fatigue properties through conducting solution treatments at high temperatures of over 800℃. Regarding this hardening mechanism, there is a solid solution hardening theory, in which the Ag-Cu α₁ phase and Pd-Cu β phase become a solid solution in the α solid solution phase that exists near the melting point, starting at 800℃, and also theories based on precipitation hardening. In light of the foregoing, we conducted heat treatment in an air atmosphere and examined the Au-Ag-Pd alloy hardening mechanism. We believe the results show that both the Ag-Cu α₁ phase and Pd-Cu β phase become a solid solution in the a solid solution phase, and theories based on precipitation hardening with regard to the precipitation of the semi-stable β' phase of the L1₀ type structure during cooling are contributing to hardening in a complex way.