The physical property and phase transformation in a commercial dental casting high gold alloy was investigated as a function of ageing temperature and time using microvickers hardness tester, X-ray diffraction, optical and electron microscopy and EPMA analyser. 1. With increasing ageing time, the hardness of solution-treated gold alloys increased slowly at the initial stage of ageing treatment at an ageing temperature of $300{sim}400^{circ}C$, and it reached a maximum value of hardness at the medium stage. Finally, it decreased gradually during further ageing. The maximum value of hardness at was similar with that of the conventional materials and suitable for using as the crown & bridge. 2. During isothermal ageing at a temperature range of $300{sim}400^{circ}C$, three phases consisting of the Au-rich ${alpha}_1$phase with a face-centered cubic structure, the Pt3Zn ${alpha}_2$phase with an ordered AuCu3(L12) type(f.c.c.) and the Pt-rich ${alpha}_3$phase with face-centered cubic structure in solution-treated gold alloys were transformed into different three phases consisting of the ${alpha}_1$phase, the ${alpha}_3$phase and the PtZn $eta$phase with an ordered AuCu I(L10) type. 3. The hardening of gold alloys was attributed to the lattice strains of the matrix resulting from the transformation of the ${alpha}_2$phase to the $eta$phase. 4. The softening of gold alloys during over-ageing was attributed to the coarsening of the nodules consisting of the $eta$phase and ${alpha}_1$matrix.