The age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental casting gold alloy composed of 56Au-25Ag-11.8Cu-5Pd-1.7Zn-0.4Pt-0.1Ir (wt.%) were elucidated by characterizing the age-hardening behaviour, phase transformations, changes in microstructure and changes in element distribution. The fast and apparent increase in hardness at the initial stage of the aging process at 400°C was caused by the nucleation and growth of the metastable Ag-Au-rich phase and the Cu-Au-rich phase by the miscibility limit of Ag and Cu. The transformation of the metastable Ag-Au-rich phase into the stable Ag-Aurich phase progressed concurrently with the ordering of the Cu-Au-rich phase into the AuCu I phase through the metastable state, which resulted in the subsequent increase in hardness. The further increase in hardness was restrained before complete decomposition of the parent α 0 phase due to the initiation of the lamellar-forming grain boundary reaction. The progress of the lamellar-forming grain boundary reaction was not directly connected with the phase transformation of the metastable phases into the final product phases. The heterogeneous expansion of the lamellar structure from the grain boundary caused greater softening than the subsequent further coarsening of the lamellar structure. The lamellar structure was composed of the Ag-Au-rich layer which was Cu-, Pd-and Zn-depleted and the AuCu I layer containing Pd and Zn.