Effects of 0.5 wt% Sn-addition to the dental casting Au-12Pt-0.6Cu alloy on the interfacial microstructures and bonding strength between porcelain and the alloy were investigated. Porcelain powders (SiO2-based oxides) are bonded through a thermal schedule consisting of preoxidation, 1st firing, and 2nd firing. Interfacial microstructures were examined after pre-oxidation and 2nd firing, respectively, by scanning and transmission electron microscopy. The bonding strength of the Au-12Pt-0.6Cu and Au-12Pt-0.6Cu-0.5Sn alloys with porcelain was about 24.6 MPa and 46.2 MPa, respectively. The higher bonding strength of the Sn-added alloy compared with that of the alloy without Sn is attributed to the SnO2 formed at the interface between porcelain and the alloy during pre-oxidation. SnO2 layer is thought to enhance chemical bonding with various oxides in the porcelain and, accordingly, improve bonding strength.