This study examined the age-hardening by miscibility limit in a multi-purpose dental gold alloy containing platinum. The hardness increased rapidly in the initial stage of the aging process, reached the maximum value, then decreased continuously with aging time. The significant hardness increase resulted from the heterogeneous precipitation of the Pt-rich β phase from the grain boundary of the Au-rich α1 matrix due to the miscibility limit of Au-Pt system. With increasing aging time, the fine Pt-rich β precipitates covered almost the whole matrix, and by further aging, the precipitates grew coarse. The microstructural coarsening reduced the interface between the Au-rich α1 matrix and the Pt-rich β precipitates, which released the lattice strains between the two phases, resulting in a softening effect. In the later stage of aging process, the Au-containing Pt3In particle-like structure was transformed into the Au-depleted particle-like structure containing relatively large amounts of Cu resulting from the overlapping miscibility limit of both Au-Pt and Ag-Cu systems, which was responsible for the slow decreasing rate in hardness in the later stage of aging.