The effects of Sn addition on the internal oxidation of Ag-Cd alloys were studied. Four types of Ag-15Cd alloys with p.3, 0.5, 0.8, snd 1.5 wt% Sn were prepared by induction melting in Ar atmosphere. Alloys were internally oxidized at 677, 735 and 782 C with 1.0-3.0×10^5 Pa oxygen partial pressure. Internal oxidation increased the hardness of all alloys by 65-100%. Alloys with higher Sn content showed higher hardness. Oxidation condition proved to have significant influence on the oxidation microstructure and wear resistance. The oxides observed after internal oxidation were identified as SnO₂, Cd₂SnO₄ and CdO. Cd₂SnO₄ particles were embedded within CdO particles. The mechanism of oxidation is considered as follows. First. Sn is oxidized to form SnO₂, then it transforms to Cd₂SnO₄ by furthier reaction with Cd and O. Finally CdO grows around the Cd₂SnO₄ particles. Because Cd₂SnO₄ particles act as nuclei far CdO precipitation, finer dispersion of oxides occurs in higher Sn content alloys resulting in higher hardness and improved wear resistance of the alloy.