The purpose of this study is to investigate the effect of small Sn additions on the microstructures and the mechanical properties of the Au-Pt-Cu alloys. The hardness of the Au-Pt-Cu-xSn alloys rapidly increased with increasing aging time at 600°C until it reached an almost constant value at aging time of 30min. In addition, the hardness of the Au-Pt-Cu-xSn (x = 0,0.1,0.3, 0.5, 1.0) alloys aged at 600°C for 30 min, rapidly increased with increasing Sn content and reached a maximum hardness at 0.5wt%, but it slightly decreased with more increasing in Sn content. This is ascribed to wide dispersion of large Pt3Sn precipitate, which is softer than matrix composed of Au-Pt-Cu-Sn solid solution. The grain size of the Au-Pt-Cu-xSn alloys decreased with increasing Sn content irrespective of the heat- and aging-treatments, but the grain size of the aged alloys was finer than that of solution treated alloys.