The effect of indium on the microstructures and mechanical properties of a Au-Pt-Cu alloy was investigated. The Au-Pt-Cu-xIn alloys heat-treated at 550°C for 30 min revealed a maximum hardness value of 207 HV, irrespective of the heat temperature and In contents. Also, the hardness of the Au-Pt-Cu-xIn alloys (x = 0.5, 1.0, 1.5, 2.0) aged at 550 °C rapidly increased with increasing aging time, and it reached an almost constant value after 30 min. The hardness of the Au- Pt-Cu-xIn alloys aged at 550°C for 30 min increased with increasing In content until 1.5wt%, but it slightly decreased with more increasing In content. Also, a variation of the tensile strength of the alloys with In contents showed a similar trend of hardness change with In contents. Analysis of EDS and TEM revealed that the microstructure of Au-Pt-Cu-xIn alloys is composed of solid solution with fcc structure and intermetallic InPt3 precipitate with L12 structure. Based on this investigation, it can be concluded that an increase in hardness of Au-Pt-Cu-xIn alloys is ascribed to a complex effect of the precipitation hardening of InPt3 and the grain size refinement.